drm/amdgpu: Move test for fbdev GEM object into generic helper

drm/amdgpu: Move test for fbdev GEM object into generic helper

Provide a generic helper that tests if fbdev emulation is backed by
a specific GEM object. Not all drivers use client buffers (yet), hence
also test against the first GEM object in the fbdev framebuffer.

Convert amdgpu. The helper will also be useful for radeon.

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