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 <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Dave Airlie <[e-mail]>
Reported-by: Stefan Agner <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>

drivers/gpu/drm/mxsfb/Kconfig

···
 	select DRM_KMS_HELPER
 	select DRM_KMS_FB_HELPER
 	select DRM_KMS_CMA_HELPER
+	select DRM_PANEL
 	help
 	  Choose this option if you have an i.MX23/i.MX28/i.MX6SX MXSFB
 	  LCD controller.