drm/amdgpu: use generic fb helpers instead of setting up AMD own's.

With the shadow buffer support from generic framebuffer emulation, it's
possible now to have runpm kicked when no update for console.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>