drm/radeon/kms: Don't overwrite crtc_gen_cntl or crtc_gen_cntl2

Don't overwritte  crtc_gen_cntl or crtc_gen_cntl2 or we may loose the
cursor. This especialy happen when changing video mode. Fix bugs:
https://bugzilla.redhat.com/show_bug.cgi?id=529146

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/radeon_legacy_crtc.c

@@ -647,12 +647,8 @@ static bool radeon_set_crtc_timing(struct drm_crtc *crtc, struct drm_display_mod
 		uint32_t crtc2_gen_cntl;
 		uint32_t disp2_merge_cntl;
 
-		/* check to see if TV DAC is enabled for another crtc and keep it enabled */
-		if (RREG32(RADEON_CRTC2_GEN_CNTL) & RADEON_CRTC2_CRT2_ON)
-			crtc2_gen_cntl = RADEON_CRTC2_CRT2_ON;
-		else
-			crtc2_gen_cntl = 0;
-
+		/* if TV DAC is enabled for another crtc and keep it enabled */
+		crtc2_gen_cntl = RREG32(RADEON_CRTC2_GEN_CNTL) & 0x00718080;
 		crtc2_gen_cntl |= ((format << 8)
 				   | RADEON_CRTC2_VSYNC_DIS
 				   | RADEON_CRTC2_HSYNC_DIS
@@ -681,7 +677,8 @@ static bool radeon_set_crtc_timing(struct drm_crtc *crtc, struct drm_display_mod
 		uint32_t crtc_ext_cntl;
 		uint32_t disp_merge_cntl;
 
-		crtc_gen_cntl = (RADEON_CRTC_EXT_DISP_EN
+		crtc_gen_cntl = RREG32(RADEON_CRTC_GEN_CNTL) & 0x00718000;
+		crtc_gen_cntl |= (RADEON_CRTC_EXT_DISP_EN
 				 | (format << 8)
 				 | RADEON_CRTC_DISP_REQ_EN_B
 				 | ((mode->flags & DRM_MODE_FLAG_DBLSCAN)