drm/amd/pm: Disable SCLK switching on Oland with high pixel clocks (v3)

drm/amd/pm: Disable SCLK switching on Oland with high pixel clocks (v3)

Port of commit 227545b9a08c ("drm/radeon/dpm: Disable sclk
switching on Oland when two 4K 60Hz monitors are connected")

This is an ad-hoc DPM fix, necessary because we don't have
proper bandwidth calculation for DCE 6.

We define "high pixelclock" for SI as higher than necessary
for 4K 30Hz. For example, 4K 60Hz and 1080p 144Hz fall into
this category.

When two high pixel clock displays are connected to Oland,
additionally disable shader clock switching, which results in
a higher voltage, thereby addressing some visible flickering.

v2:
Add more comments.
v3:
Split into two commits for easier review.

Fixes: 841686df9f7d ("drm/amdgpu: add SI DPM support (v4)")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>