drm: ditch strange DRIVER_DMA_QUEUE only error bail-out

Only one driver (i810) even sets that flag. Now the actual locking
code uncoditionally promotes lock->context to an unsigned int.

Closer inspection of the userspace reveals that the drm lock context
is defined as an unsigned int (at least on linux). I suspect we just
have a strange case of signedness confusion going on.

Tested on my i815, doesn't seem to break anything.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>