drm/mga200g: Use drm_fb_helper_fill_info

Only changes the name of the fb from "mgadrmfb" to "mga200drmfb".

v2: Rebase

v3: Note the name change (Noralf)

Acked-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: Junwei Zhang <Jerry.Zhang@amd.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326132008.11781-12-daniel.vetter@ffwll.ch