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>

drivers/gpu/drm/Makefile

@@ -21,6 +21,8 @@ drm-$(CONFIG_DRM_PANEL) += drm_panel.o
 drm-$(CONFIG_OF) += drm_of.o
 drm-$(CONFIG_AGP) += drm_agpsupport.o
 
+drm-y += $(drm-m)
+
 drm_kms_helper-y := drm_crtc_helper.o drm_dp_helper.o drm_probe_helper.o \
 		drm_plane_helper.o drm_dp_mst_topology.o drm_atomic_helper.o
 drm_kms_helper-$(CONFIG_DRM_LOAD_EDID_FIRMWARE) += drm_edid_load.o