drm/radeon/dpm: disable mclk switching on desktop RV770

Mclk switching doesn't seem to work reliably on these
cards.  Most RV770 boards specify the same mclk for all
performance levels anyway so in most cases, this has
no affect.

Bug:
https://bugs.freedesktop.org/show_bug.cgi?id=73067

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/radeon/rv770_dpm.c

@@ -2536,6 +2536,12 @@ bool rv770_dpm_vblank_too_short(struct radeon_device *rdev)
 	    (rdev->pdev->subsystem_device == 0x1c42))
 		switch_limit = 200;
 
+	/* RV770 */
+	/* mclk switching doesn't seem to work reliably on desktop RV770s */
+	if ((rdev->family == CHIP_RV770) &&
+	    !(rdev->flags & RADEON_IS_MOBILITY))
+		switch_limit = 0xffffffff; /* disable mclk switching */
+
 	if (vblank_time < switch_limit)
 		return true;
 	else