drm/mga: remove device_is_agp callback

[ Upstream commit 858b2c1bf820ebfba89c5e2867ab882bdb5b2f5a ]

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


Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>