drm/gma500: Use drm_fb_helper_fill_info

This will change the fb name from "psbdrmfb" to "gma500drmfb".

v2: Rebase

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: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326132008.11781-9-daniel.vetter@ffwll.ch