gbm: Search LIBGL_DRIVERS_PATH if GBM_DRIVERS_PATH is not set

gbm: Search LIBGL_DRIVERS_PATH if GBM_DRIVERS_PATH is not set

The GBM_DRIVERS_PATH environment variable is not documented, and only
used to set the location of gbm drivers, while LIBGL_DRIVERS_PATH is
used for everything else, and is documented.

Generally this split leads to confusion as to why gbm doesn't work.

This patch will read LIBGL_DRIVERS_PATH as a fallback if
GBM_DRIVERS_PATH is not set.

The comments clearly indicate that using LIBGL_DRIVERS_PATH is
preferred over GBM_DRIVERS_PATH.

v2: - Use GBM_DRIVERS_PATH as a fallback
v3: [jordan.l.justen@intel.com] - Make LIBGL_DRIVERS_PATH the fallback

Signed-off-by: Dylan Baker <baker.dylan.c@gmail.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>