This restricts the use of the big kernel lock to the i830 and i810
device drivers. The three remaining users in common code (open, ioctl
and release) get converted to a new mutex, the drm_global_mutex,
making the locking stricter than the big kernel lock.

This may have a performance impact, but only in those cases that
currently don't use DRM_UNLOCKED flag in the ioctl list and would
benefit from that anyway.

The reason why i810 and i830 cannot use drm_global_mutex in their
mmap functions is a lock-order inversion problem between the current
use of the BKL and mmap_sem in these drivers. Since the BKL has
release-on-sleep semantics, it's harmless but it would cause trouble
if we replace the BKL with a mutex.

Instead, these drivers get their own ioctl wrappers that take the
BKL around every ioctl call and then set their own handlers as
DRM_UNLOCKED.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Dave Airlie <airlied@redhat.com>