drm/amdgpu: Remove unused function amdgpu_bo_fbdev_mmap()

Remove an unused function. Mapping the fbdev framebuffer is apparently
not supported.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>