drm/amdgpu: Use drm_fb_helper_fill_info

Should not cause any changes.

v2: Rebase

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: Samuel Li <Samuel.Li@amd.com>
Cc: "Michel Dänzer" <michel.daenzer@amd.com>
Cc: Huang Rui <ray.huang@amd.com>
Cc: Junwei Zhang <Jerry.Zhang@amd.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Daniel Stone <daniels@collabora.com>
Cc: Shirish S <shirish.s@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326132008.11781-4-daniel.vetter@ffwll.ch