staging: imx-drm: Fix modular build of DRM_IMX_IPUV3

commit b8d181e4

 (staging: drm/imx: add drm plane support) added a file
to the make target for DRM_IMX_IPUV3 but didn't adjust the objs required
to actually build that as a module.  Kbuild got confused and this lead to
link errors like:

ERROR: "ipu_plane_disable" [drivers/staging/imx-drm/ipuv3-crtc.ko] undefined!
ERROR: "ipu_plane_enable" [drivers/staging/imx-drm/ipuv3-crtc.ko] undefined!

Additionally, it added a call to imx_drm_crtc_id which also fails with a
link error as above.  To fix this, we adjust the make target with the proper
objs, which will change the name of the resulting .ko.  We also add an
EXPORT_SYMBOL_GPL for imx_drm_crtc_id.

Signed-off-by: Josh Boyer <jwboyer@fedoraproject.org>
Fixes: b8d181e4

 '(staging: drm/imx: add drm plane support)'
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>