drm/amdgpu: use drm_mode_vrefresh() rather than mode->vrefresh

BugLink: http://bugs.launchpad.net/bugs/1590455

commit 6b8812eb upstream.

This is a port of radeon commit:
3d2d98ee
drm/radeon: use drm_mode_vrefresh() rather than mode->vrefresh
to amdgpu.
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_dpm.c

@@ -150,7 +150,7 @@ u32 amdgpu_dpm_get_vrefresh(struct amdgpu_device *adev)
 		list_for_each_entry(crtc, &dev->mode_config.crtc_list, head) {
 			amdgpu_crtc = to_amdgpu_crtc(crtc);
 			if (crtc->enabled && amdgpu_crtc->enabled && amdgpu_crtc->hw_mode.clock) {
-				vrefresh = amdgpu_crtc->hw_mode.vrefresh;
+				vrefresh = drm_mode_vrefresh(&amdgpu_crtc->hw_mode);
 				break;
 			}
 		}