i915: Allow allocating ARGB buffers for scanout

i915: Allow allocating ARGB buffers for scanout

CL:991261 changed our primary framebuffer format from XRGB to ARGB. We
still only scanout as XRGB where the display controller doesn't
support ARGB, but minigbm doesn't know that. As a result, when we try
to allocate an ARGB buffer with the SCANOUT useflag, we get a linear
buffer.  This breaks PSR and regresses performance and, in general,
just isn't what we want.

This CL enables SCANOUT for all ARGB formats where the corresponding
XRGB formats supports scanout. In the end, it's up to the caller to
make sure a format works for scanout anyway.

BUG=827188,830969
TEST=test_that graphics_Idle on samus

Change-Id: Iab428e5b21abedcac7cee86fccc8df50dab3f471
Reviewed-on: https://chromium-review.googlesource.com/1008867
Commit-Ready: Kristian H. Kristensen <hoegsberg@chromium.org>
Tested-by: Kristian H. Kristensen <hoegsberg@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>