drm/amd/display: Compare mpcc_inst to mpcc_count instead of a constant

[Why]
This assert triggers a false negative because there are more than 4 MPCCs
on many asics.

[How]
 - change assert comparisson
 - remove unused variable

Signed-off-by: Joshua Aberback <joshua.aberback@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>