drm/amdgpu/vce1: Workaround PLL timeout on FirePro W9000

Sometimes the VCE PLL times out waiting for CTLACK/CTLACK2.
When it happens, the VCE still works, but much slower.
Observed on a Tahiti GPU, but not all:
- FirePro W9000 has the issue
- Radeon R9 280X not affected
- Radeon HD 7990 not affected

As a workaround, on the affected chip just don't put the
VCE PLL in sleep mode. Leaving the VCE PLL in bypass mode
or reset mode both work. Using bypass mode is simpler.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -1918,6 +1918,14 @@ static int si_set_vce_clocks(struct amdgpu_device *adev, u32 evclk, u32 ecclk)
 		     ~VCEPLL_BYPASS_EN_MASK);
 
 	if (!evclk || !ecclk) {
+		/*
+		 * On some chips, the PLL takes way too long to get out of
+		 * sleep mode, causing a timeout waiting on CTLACK/CTLACK2.
+		 * Leave the PLL running in bypass mode.
+		 */
+		if (adev->pdev->device == 0x6780)
+			return 0;
+
 		/* Keep the Bypass mode, put PLL to sleep */
 		WREG32_SMC_P(CG_VCEPLL_FUNC_CNTL, VCEPLL_SLEEP_MASK,
 			     ~VCEPLL_SLEEP_MASK);