drm/bridge: Build the panel wrapper in drm_kms_helper

This fixes the following depmod error when building drm as a module:
depmod: ERROR: Found 6 modules in dependency cycles!
depmod: ERROR: Cycle detected: drm -> drm_kms_helper -> drm

Fixes: 13dfc054 ("drm/bridge: Refactor out the panel wrapper from the lvds-encoder bridge.")
Tested-by: Lofstedt, Marta <marta.lofstedt@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/3fd262cf-1db6-4335-320c-af92f9014502@linux.intel.com