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/drm_pci.c b/drivers/gpu/drm/drm_pci.c
index 60af894..a3b356e 100644
--- a/drivers/gpu/drm/drm_pci.c
+++ b/drivers/gpu/drm/drm_pci.c
@@ -191,7 +191,7 @@ int drm_irq_by_busid(struct drm_device *dev, void *data,
 static void drm_pci_agp_init(struct drm_device *dev)
 {
 	if (drm_core_check_feature(dev, DRIVER_USE_AGP)) {
-		if (drm_pci_device_is_agp(dev))
+		if (pci_find_capability(dev->pdev, PCI_CAP_ID_AGP))
 			dev->agp = drm_agp_init(dev);
 		if (dev->agp) {
 			dev->agp->agp_mtrr = arch_phys_wc_add(