drm/i915: Add missing dependency on Intel AGP support.

Users could accidentally enable AGP but not the Intel AGP support, and get
a DRM that doesn't probe as a result.

Bug #22358.
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/Kconfig

@@ -81,6 +81,7 @@ config DRM_I830
 
 config DRM_I915
 	tristate "i915 driver"
+	depends on AGP_INTEL
 	select FB_CFB_FILLRECT
 	select FB_CFB_COPYAREA
 	select FB_CFB_IMAGEBLIT