gpu: drm: radeon: radeon_object: Remove unused function

Remove the function radeon_bo_fbdev_mmap() that is not used anywhere.

This was partially found by using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>