drm/amd/display: Cover edge-case when changing DISPCLK WDIVIDER

[WHY]
When changing the DISPCLK_WDIVIDER value from 126 to 127, the change in
clock rate is too great for the FIFOs to handle. This can cause visible
corruption during clock change.

HW has handed down this register sequence to fix the issue.

[HOW]
The sequence, from HW:
a.	127 -> 126
Read  DIG_FIFO_CAL_AVERAGE_LEVEL
FIFO level N = DIG_FIFO_CAL_AVERAGE_LEVEL / 4
Set DCCG_FIFO_ERRDET_OVR_EN = 1
Write 1 to OTGx_DROP_PIXEL for (N-4) times
Set DCCG_FIFO_ERRDET_OVR_EN = 0
Write DENTIST_DISPCLK_RDIVIDER = 126

Because of frequency stepping, sequence a can be executed to change the
divider from 127 to any other divider value.

b.	126 -> 127
Read  DIG_FIFO_CAL_AVERAGE_LEVEL
FIFO level N = DIG_FIFO_CAL_AVERAGE_LEVEL / 4
Set DCCG_FIFO_ERRDET_OVR_EN = 1
Write 1 to OTGx_ADD_PIXEL for (12-N) times
Set DCCG_FIFO_ERRDET_OVR_EN = 0
Write DENTIST_DISPCLK_RDIVIDER = 127

Because of frequency stepping, divider must first be set from any other
divider value to 126 before executing sequence b.

Signed-off-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>