drm/fb-helper: don't call remove_conflicting_framebuffers for FB=m && DRM=y

drm/fb-helper: don't call remove_conflicting_framebuffers for FB=m && DRM=y

When CONFIG_DRM_KMS_FB_HELPER is disabled, we can have a configuration
in which some DRM drivers are built-in, but the framebuffer core is a
loadable module. This results in a link error, such as:

drivers/gpu/drm/radeon/radeon.o: In function `radeon_pci_probe':
radeon_kfd.c:(.text.radeon_pci_probe+0xbc): undefined reference to `remove_conflicting_framebuffers'
drivers/gpu/drm/amd/amdgpu/amdgpu.o: In function `amdgpu_pci_probe':
amdgpu_mn.c:(.text.amdgpu_pci_probe+0xa8): undefined reference to `remove_conflicting_framebuffers'
drivers/gpu/drm/mgag200/mgag200.o: In function `mga_vram_init':
mgag200_ttm.c:(.text.mga_vram_init+0xa8): undefined reference to `remove_conflicting_framebuffers'
drivers/gpu/drm/mgag200/mgag200.o: In function `mga_pci_probe':
mgag200_ttm.c:(.text.mga_pci_probe+0x88): undefined reference to `remove_conflicting_framebuffers'
Makefile:969: recipe for target 'vmlinux' failed

This changes the compile-time check to IS_REACHABLE, which means we end up
not calling remove_conflicting_framebuffers() in the configuration, which
seems good enough, as we know that no framebuffer driver is loaded by the
time that the built-in DRM driver calls remove_conflicting_framebuffers.

We could alternatively avoid the link error by forcing CONFIG_FB to not
be a module in this case, but that wouldn't change anything at runtime,
and just make the already convoluted set of dependencies worse here.

I could not find out what happens if the fbdev driver gets loaded as
a module after the DRM driver is already initialized, but that is a case
that can happen with or without this patch.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 0a3bfe29f816 ("drm/fb-helper: Fix the dummy remove_conflicting_framebuffers")
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20160829123428.3260105-1-arnd@arndb.de
Link: http://patchwork.freedesktop.org/patch/msgid/1472461923-14364-1-git-send-email-gnuiyl@gmail.com