drm/i915: Add dependency on the intel agp module

See http://bugzilla.kernel.org/show_bug.cgi?id=15021



Make sure that the appropriate AGP module is loaded and probed before
trying to set up the DRM.  The DRM already depends on the AGP core,
but in this case we know the specific AGP driver we need too, and can
help users avoid the trap of loading the AGP driver after the DRM
driver.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>