drm/i915: page table generalizations

No functional changes, but will improve code clarity and removed some
duplicated defines.

Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --cc drivers/gpu/drm/i915/i915_gem_gtt.c
Simple merge

diff --cc drivers/gpu/drm/i915/i915_gem_gtt.h
Simple merge