drm/amd/display: Initialize num_pkrs on VANGOGH.

As far a I can tell uses a variant of DCN3xx which uses num_pkrs.

If we do not initialize the variable we will set the register field
to ilog2(0) = -1, though the mask will reduce that to 7. Pretty sure
7 is not the value we want here.

Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-and-Tested-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>