drm/radeon: add pageflip hooks for fusion

Looks like these got passed over with both being merged at the same
time but not quite meeting in the middle.

should fix: https://bugs.freedesktop.org/show_bug.cgi?id=34137
along with Michael's phoronix article.

Reported-by: Chi-Thanh Christopher Nguyen
Article-written-by: Michael Larabel @ phoronix
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/radeon_asic.c

@@ -834,6 +834,9 @@ static struct radeon_asic sumo_asic = {
 	.pm_finish = &evergreen_pm_finish,
 	.pm_init_profile = &rs780_pm_init_profile,
 	.pm_get_dynpm_state = &r600_pm_get_dynpm_state,
+	.pre_page_flip = &evergreen_pre_page_flip,
+	.page_flip = &evergreen_page_flip,
+	.post_page_flip = &evergreen_post_page_flip,
 };
 
 static struct radeon_asic btc_asic = {