drm/radeon/kms/rs4xx: make sure crtcs are enabled when setting timing

based on ddx patch from Matthias Hopf.
Signed-off-by: NAlex Deucher <alexdeucher@gmail.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/radeon_legacy_crtc.c

@@ -603,6 +603,10 @@ static bool radeon_set_crtc_timing(struct drm_crtc *crtc, struct drm_display_mod
 				      ? RADEON_CRTC2_INTERLACE_EN
 				      : 0));
 
+		/* rs4xx chips seem to like to have the crtc enabled when the timing is set */
+		if ((rdev->family == CHIP_RS400) || (rdev->family == CHIP_RS480))
+			crtc2_gen_cntl |= RADEON_CRTC2_EN;
+
 		disp2_merge_cntl = RREG32(RADEON_DISP2_MERGE_CNTL);
 		disp2_merge_cntl &= ~RADEON_DISP2_RGB_OFFSET_EN;
 
@@ -630,6 +634,10 @@ static bool radeon_set_crtc_timing(struct drm_crtc *crtc, struct drm_display_mod
 				    ? RADEON_CRTC_INTERLACE_EN
 				    : 0));
 
+		/* rs4xx chips seem to like to have the crtc enabled when the timing is set */
+		if ((rdev->family == CHIP_RS400) || (rdev->family == CHIP_RS480))
+			crtc_gen_cntl |= RADEON_CRTC_EN;
+
 		crtc_ext_cntl = RREG32(RADEON_CRTC_EXT_CNTL);
 		crtc_ext_cntl |= (RADEON_XCRT_CNT_EN |
 				  RADEON_CRTC_VSYNC_DIS |