drm/radeon/dpm: implement vblank_too_short callback for evergreen

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.h b/drivers/gpu/drm/radeon/radeon_asic.h
index ca219e9..0ef8b49 100644
--- a/drivers/gpu/drm/radeon/radeon_asic.h
+++ b/drivers/gpu/drm/radeon/radeon_asic.h
@@ -545,6 +545,7 @@
 int cypress_dpm_set_power_state(struct radeon_device *rdev);
 void cypress_dpm_display_configuration_changed(struct radeon_device *rdev);
 void cypress_dpm_fini(struct radeon_device *rdev);
+bool cypress_dpm_vblank_too_short(struct radeon_device *rdev);
 int btc_dpm_init(struct radeon_device *rdev);
 void btc_dpm_setup_asic(struct radeon_device *rdev);
 int btc_dpm_enable(struct radeon_device *rdev);