drm/mga: remove device_is_agp callback

It's only for a device quirk, and we might as well do that in the load
callback.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170125062657.19270-10-daniel.vetter@ffwll.ch