drm/i810: drop device_is_agp callback

Use the same trick we used for i915 when we still had ums support:
Just initialize the agp support unconditionally in the driver load
function.

Unfortunately that means we need to export drm_agp_init again, but I
think that's a lesser evil.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170125062657.19270-9-daniel.vetter@ffwll.ch