drm/amd/display: remove unused matching_stream_ptrs variable

clang with W=1 reports
drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link_enc_cfg.c:625:6: error:
  variable 'matching_stream_ptrs' set but not used [-Werror,-Wunused-but-set-variable]
        int matching_stream_ptrs = 0;
            ^
This variable is not used so remove it.

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>