drm: remove device_is_agp callback

With that the drm_pci_device_is_agp function becomes trivial, so
inline that too. And while at it, move the drm_pci_agp_destroy
declaration into drm-internal.h, since it's not used by drivers.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170125062657.19270-11-daniel.vetter@ffwll.ch