drm/gma500: Nuke device_is_agp callback

Returning 0 for an on-chip gpu doesn't change anything at all.

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170125062657.19270-8-daniel.vetter@ffwll.ch