drm/amd/display: Remove unused MaxUsedBW variable

Remove the variable MaxUsedBW from the function
DISPCLKDPPCLKDCFCLKDeepSleepPrefetchParametersWatermarksAndPerformanceCalculation.
As a side-effect, the variables MaxPerPlaneVActiveWRBandwidth and
WRBandwidth are also removed.

This was pointed by clang with the following warning:

drivers/gpu/drm/amd/amdgpu/../display/dc/dml/dcn30/display_mode_vba_30.c:3043:10:
warning: variable 'MaxUsedBW' set but not used [-Wunused-but-set-variable]
                double MaxUsedBW = 0;
                       ^
1 warning generated.

Reviewed-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>