drm/i915: Silence sparse by using gfp_t

Sparse enforces that GFP flags are only manipulated inside gfp_t locals.

Fixes: 4d470f73 ("drm/i915: Avoid undefined behaviour of "u32 >> 32"")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170901145729.21363-1-chris@chris-wilson.co.uk


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>