As detect will use hw registers and may modify structures, it needs to be
serialised by use of the dev->mode_config.mutex. Make it so.

Otherwise, we may cause random crashes as the sysfs file is queried
whilst a concurrent hotplug poll is being run. For example:

[ 1189.189626] BUG: unable to handle kernel NULL pointer dereference at 00000100
[ 1189.189821] IP: [<e0c22019>] intel_tv_detect_type+0xa2/0x203 [i915]
[ 1189.190020] *pde = 00000000
[ 1189.190104] Oops: 0000 [#1] SMP
[ 1189.190209] last sysfs file: /sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-SVIDEO-1/status
[ 1189.190412] Modules linked in: mperf cpufreq_conservative cpufreq_userspace cpufreq_powersave cpufreq_stats decnet uinput fuse loop joydev snd_hd a_codec_realtek snd_hda_intel snd_hda_codec snd_hwdep snd_pcm_oss snd_mixer_oss snd_pcm i915 snd_seq_midi snd_rawmidi snd_seq_midi_event snd_seq drm_kms_helper snd_timer uvcvideo d rm snd_seq_device eeepc_laptop tpm_tis usbhid videodev i2c_algo_bit v4l1_compat snd sparse_keymap i2c_core hid serio_raw tpm psmouse evdev tpm_bios rfkill shpchp ac processor rng_c ore battery video power_supply soundcore pci_hotplug button output snd_page_alloc usb_storage uas ext3 jbd mbcache sd_mod crc_t10dif ata_generic ahci libahci ata_piix libata uhci_h cd ehci_hcd scsi_mod usbcore thermal atl2 thermal_sys nls_base [last unloaded: scsi_wait_scan]
[ 1189.192007]
[ 1189.192007] Pid: 1464, comm: upowerd Not tainted 2.6.37-2-686 #1 ASUSTeK Computer INC. 701/701
[ 1189.192007] EIP: 0060:[<e0c22019>] EFLAGS: 00010246 CPU: 0
[ 1189.192007] EIP is at intel_tv_detect_type+0xa2/0x203 [i915]
[ 1189.192007] EAX: 00000000 EBX: dca74000 ECX: e0f68004 EDX: 00068004
[ 1189.192007] ESI: dd110c00 EDI: 400c0c37 EBP: dca7429c ESP: de365e2c
[ 1189.192007]  DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
[ 1189.192007] Process upowerd (pid: 1464, ti=de364000 task=dcc8acb0 task.ti=de364000)
[ 1189.192007] Stack: Mar 15 03:43:23 hostname kernel: [ 1189.192007]  e0c2cda4 70000000 400c0c30 00000000 dd111000 de365e54 de365f24 dd110c00
[ 1189.192007]  e0c22203 01000000 00000003 00000000 00000000 00000000 00000000 4353544e
[ 1189.192007]  30383420 00000069 00000000 00000000 00000000 00000000 00000000 00000000
[ 1189.192007] Call Trace: Mar 15 03:43:23 hostname kernel: [ 1189.192007]  [<e0c22203>] ?  intel_tv_detect+0x89/0x12d [i915]
[ 1189.192007]  [<e0a9dcef>] ?  status_show+0x0/0x2f [drm]
[ 1189.192007]  [<e0a9dd03>] ?  status_show+0x14/0x2f [drm]

[Digression: what is upowerd doing reading those power hungry files?]
Reported-by: Paul Menzel <paulepanter@users.sourceforge.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>