i965: Sync i915_drm.h from kernel for IMG_context_priority

i965: Sync i915_drm.h from kernel for IMG_context_priority

Pulling in changes up to

    kernel commit ac14fbd460d0ec16e7750e40dcd8199b0ff83d0a
    Author: Chris Wilson <chris@chris-wilson.co.uk>
    Date:   Tue Oct 3 21:34:53 2017 +0100

	drm/i915/scheduler: Support user-defined priorities

and including the fixup from

    kernel commit 822a4b673284672af697ccd66e8795f8a712a90d
    Author: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
    Date:   Fri Oct 6 13:45:59 2017 +0300

	drm/i915: Don't use BIT() in UAPI section

for implementing IMG_context_priority.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>