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

Add a function for drivers to check if the a firmware initialized
fb is corresponds to their aperture.  This allows drivers to check if the
device corresponds to what the firmware set up as the display device.

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=215203
Bug: drm/amd#1840


Signed-off-by: Alex Deucher <alexander.deucher@amd.com>