drm: Pretty print pixel format in drm_fb_get_bpp_depth() and format_check()

drm_fb_get_bpp_depth() likes to complain about unsupported pixel formats
but doesn't bother telling us what the format was. Also format_check()
just returns an error when it encouters an invalid format, leaving the
user scratching his head trying to figure out why addfb failed. Make
life a bit easier by using drm_get_format_name() in both places.

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