drm: Document that drm_panel_orientation_quirks.c is shared with fbdev

Add a comment to the DRM_PANEL_ORIENTATION_QUIRKS documenting that the
reason for a separate Kconfig for this is because
drm_panel_orientation_quirks.c code is shared with fbdev.
Suggested-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171204223321.15799-1-hdegoede@redhat.com

drivers/gpu/drm/Kconfig

@@ -27,6 +27,7 @@ config DRM_MIPI_DSI
 	bool
 	depends on DRM
 
+# Separate option because drm_panel_orientation_quirks.c is shared with fbdev
 config DRM_PANEL_ORIENTATION_QUIRKS
 	tristate