egl/drm: ensure the backing gbm is set before using it

Currently, if we error out before gbm_dri is set (say due to a different
name of the backing GBM implementation, or otherwise) the tear down will
trigger a NULL ptr deref and crash out.

Move the gbm_dri initialization as early as possible.

v2: Drop check in dri2_teardowm_drm (Eric)

Reported-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit 72b97ad9)