minigbm: cros_gralloc: Use pkg-config for LIBS

minigbm: cros_gralloc: Use pkg-config for LIBS

This changes the Makefile to use pkg-config to query for necessary
library paths and names instead of hardcoding -ldrm.

BUG=b:36540057
TEST=emerge-reef arc-cros-gralloc with multilib patches
CQ-DEPEND=CL:505794

Change-Id: Id223203506d71b3966e9730f92badb461eb6cb1b
Reviewed-on: https://chromium-review.googlesource.com/499971
Commit-Ready: Tomasz Figa <tfiga@chromium.org>
Tested-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: Luis Hector Chavez <lhchavez@chromium.org>
Reviewed-by: Nicolas Boichat <drinkcat@chromium.org>