drm/edid: Rewrite drm_mode_find_dmt search loop

No functional change, but will make an upcoming change clearer.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Tested-by: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>