i915: Add GBM_BO_USE_HW_VIDEO_DECODER use flag

i915: Add GBM_BO_USE_HW_VIDEO_DECODER use flag

This flag is used to indicate that the platform video decoder will be
writing into this buffer and that it should be allocated
accordingly. On Intel, this means that we have to allocate y-tiled
NV12 for libva to be able to decode to the buffer.

We force gralloc NV12 allocations to be linear for now, since ARC++
doesn't properly pass modifiers to ChromeOS.

BUG=822346
TEST=test_that graphics_Gbm

Change-Id: I840c30d22355d26816df718b49717407e2e4620f
Reviewed-on: https://chromium-review.googlesource.com/996648
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Kristian H. Kristensen <hoegsberg@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@chromium.org>