backend/drm: fix GBM format mismatch
We create the EGL config with GBM_FORMAT_ARGB8888, but then initialize GBM BOs with GBM_FORMAT_XRGB8888. This mismatch confuses Mesa.
Instead, we can always use GBM_FORMAT_ARGB8888, and use DRM_FORMAT_XRGB8888 when calling drmModeAddFB2.