media: omap2: fix compile-testing with FB_OMAP2=m

Compile-testing with FB_OMAP2=m results in a link error:

drivers/media/platform/omap/omap_vout.o: In function `vidioc_streamoff':
omap_vout.c:(.text+0x1028): undefined reference to `omap_dispc_unregister_isr'
drivers/media/platform/omap/omap_vout.o: In function `omap_vout_release':
omap_vout.c:(.text+0x1330): undefined reference to `omap_dispc_unregister_isr'
drivers/media/platform/omap/omap_vout.o: In function `vidioc_streamon':
omap_vout.c:(.text+0x2dd4): undefined reference to `omap_dispc_register_isr'
drivers/media/platform/omap/omap_vout.o: In function `omap_vout_remove':

In order to enable compile-testing but still keep the correct dependency,
this changes the Kconfig logic so we only allow CONFIG_COMPILE_TEST
building when FB_OMAP is completely disabled, or have use the old
dependency on FB_OMAP to ensure VIDEO_OMAP2_VOUT is also a loadable
module when FB_OMAP2 is.

Fixes: d8555fd2 ("media: omap2: allow building it with COMPILE_TEST && DRM_OMAP")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>