minigbm: cros_gralloc: Use pkg-config

minigbm: cros_gralloc: Use pkg-config

This change uses pkg-config to find the libdrm headers instead of
relying on the arc-toolchain sysroot.

BUG=b:26864637
TEST=reef compiles, surfaceflinger is happy
CQ-DEPEND=CL:494886

Change-Id: Ic6142424ff5593ab5552dd61b8ae4b7dc38497fc
Reviewed-on: https://chromium-review.googlesource.com/495267
Commit-Ready: Luis Hector Chavez <lhchavez@chromium.org>
Tested-by: Luis Hector Chavez <lhchavez@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>