drm/amdgpu: clean framebuffer with GPU

CPU is not efficient to clean framebuffer especially under
virtualization, then loading driver takes long time which causes
timeout of mailbox handshake.

Signed-off-by: Pixel Ding <Pixel.Ding@amd.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>