drm/amd/pm: Treat zero vblank time as too short in si_dpm (v3)

drm/amd/pm: Treat zero vblank time as too short in si_dpm (v3)

Some parts of the code base expect that MCLK switching is turned
off when the vblank time is set to zero.

According to pp_pm_compute_clocks the non-DC code has issues
with MCLK switching with refresh rates over 120 Hz.

v3:
Add code comment to explain this better.
Add an if statement instead of changing the switch_limit.

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>