drm/radeon/dpm: implement vblank_too_short callback for 7xx

Check if we can switch the mclk during the vblank time otherwise
we may get artifacts on the screen when the mclk changes.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
diff --git a/drivers/gpu/drm/radeon/radeon_asic.c b/drivers/gpu/drm/radeon/radeon_asic.c
index a6906d4..c3b2f96 100644
--- a/drivers/gpu/drm/radeon/radeon_asic.c
+++ b/drivers/gpu/drm/radeon/radeon_asic.c
@@ -1394,6 +1394,7 @@
 		.print_power_state = &rv770_dpm_print_power_state,
 		.debugfs_print_current_performance_level = &rv770_dpm_debugfs_print_current_performance_level,
 		.force_performance_level = &rv770_dpm_force_performance_level,
+		.vblank_too_short = &rv770_dpm_vblank_too_short,
 	},
 	.pflip = {
 		.pre_page_flip = &rs600_pre_page_flip,