drm: change drm_err return type to void

The return value is not used by callers of this function
nor by uses of the DRM_ERROR macro so change the function
to return void.

Signed-off-by: Joe Perches <joe@perches.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>