drm: Make drm_fb_cma_describe() static

This function is only used in drm_fb_cma_helper.c.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>