drm: Nuke the useless 'ret' variable from drm_mode_convert_umode()

No need to store the return value in a variable since we don't have to
do any unwinding.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180313150759.27620-1-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>