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
diff --git a/drivers/gpu/drm/nouveau/nouveau_abi16.c b/drivers/gpu/drm/nouveau/nouveau_abi16.c
index 7bd4683..4df4f6e 100644
--- a/drivers/gpu/drm/nouveau/nouveau_abi16.c
+++ b/drivers/gpu/drm/nouveau/nouveau_abi16.c
@@ -199,7 +199,7 @@
 		if (!nvxx_device(device)->func->pci)
 			getparam->value = 3;
 		else
-		if (drm_pci_device_is_agp(dev))
+		if (pci_find_capability(dev->pdev, PCI_CAP_ID_AGP))
 			getparam->value = 0;
 		else
 		if (!pci_is_pcie(dev->pdev))