Revert "fbdev: fbmem: add a helper to determine if an aperture is used by a fw fb"

This reverts commit 9a45ac23.

This was added a helper for amdgpu to workaround a runtime pm regression
caused by a runtime pm fix in efifb.  We now have a better workaround
in amdgpu in
commit f95af4a9 ("drm/amdgpu: don't runtime suspend if there are displays attached (v3)")
so this workaround is no longer necessary.  Since amdgpu was the only
user of this interface, we can remove it.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>