drm: mxsfb: Select DRM_PANEL

Select DRM_PANEL, since the MXSFB driver depends on it. Otherwise,
we get the following error when compiling:

drivers/built-in.o: In function `mxsfb_probe':
core.c:(.text+0x9ce9c): undefined reference to `drm_panel_attach'
core.c:(.text+0x9cff0): undefined reference to `drm_panel_detach'
drivers/built-in.o: In function `mxsfb_panel_connector_destroy':
core.c:(.text+0x9d614): undefined reference to `drm_panel_detach'
drivers/built-in.o: In function `mxsfb_create_output':
core.c:(.text+0x9d68c): undefined reference to `of_drm_find_panel'
make: *** [Makefile:962: vmlinux] Error 1

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dave Airlie <airlied@redhat.com>
Reported-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>