drm/amd/pm: remove unused num_of_active_display variable

clang with W=1 reports
drivers/gpu/drm/amd/amdgpu/../pm/swsmu/amdgpu_smu.c:1700:6: error: variable
  'num_of_active_display' set but not used [-Werror,-Wunused-but-set-variable]
        int num_of_active_display = 0;
            ^
This variable is not used so remove it.

Fixes: 75145aab ("drm/amdgpu/swsmu: clean up a bunch of stale interfaces")
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>