drm: Hack around CONFIG_AGP=m build failures

Surprisingly kbuild can't cope with tristates in the
<module>-$(CONFIG_FOO) pattern. This patch hacks up a solution.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Michal Marek <mmarek@suse.com>
Cc: linux-kbuild@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>