drm: make of_drm_find_panel also depend on CONFIG_DRM_PANEL

For drm_of_find_panel_or_bridge() added in the next commit, an empty
version of of_drm_find_panel is needed for !CONFIG_DRM_PANEL.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Sean Paul <seanpaul@chromium.org>