x86: i915 needs pgprot_writecombine() and is_io_mapping_possible()

Impact: build fix

Theodore Ts reported that the i915 driver needs these symbols:

 ERROR: "pgprot_writecombine" [drivers/gpu/drm/i915/i915.ko] undefined!
 ERROR: "is_io_mapping_possible" [drivers/gpu/drm/i915/i915.ko] undefined!

Reported-by: Theodore Ts'o <tytso@mit.edu> wrote:
Signed-off-by: Ingo Molnar <mingo@elte.hu>