drm/i915: Stop depending upon CONFIG_AGP_INTEL

The AGP_INTEL driver provides an interface for very old userspace to
control the GART (though the GART itself was only ever emulated on Intel
systems). The pci bridge discovery code is also used by the i915.ko
driver to set up the GTT on old systems, but it does not require the
old userspace interface. When i915.ko selects the old interface, it
binds another user to the core GTT routines, and in particular creates a
second reference to the scratch pages allocated. This hinders resource
leak debugging for when we unload i915.ko as we want to assert that all
DMA pages have been released, but we appear to leak because of the
secondary interface which persists after i915.ko unloads.

All i915.ko users do not require the old /dev/agpgart interface so stop
selecting it and simplify our debugging by dropping the historical
baggage.

Note that by selecting AGP=n it was already possible to unselect
AGP_INTEL. But since we've dropped support for any of the AGP stuff
long ago there's really no point for this any more.

Also note that we still need INTEL_GTT, which is the underlying,
shared, driver for the graphics GART on gen1-5.

v2: Entirely new commit message (Chris, Ville).

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1453901881-26425-2-git-send-email-daniel.vetter@ffwll.ch

drivers/gpu/drm/i915/Kconfig

@@ -2,9 +2,7 @@ config DRM_I915
 	tristate "Intel 8xx/9xx/G3x/G4x/HD Graphics"
 	depends on DRM
 	depends on X86 && PCI
-	depends on (AGP || AGP=n)
 	select INTEL_GTT
-	select AGP_INTEL if AGP
 	select INTERVAL_TREE
 	# we need shmfs for the swappable backing store, and in particular
 	# the shmem_readpage() which depends upon tmpfs