drm/amd/display: clean-up dead code in dml2_mall_phantom

drm/amd/display: clean-up dead code in dml2_mall_phantom

pipe_idx in funtion dml2_svp_validate_static_schedulabilit, although set
is never actually used. While building with GCC 16 this gives a warning:

drivers/gpu/drm/amd/amdgpu/../display/dc/dml2/dml2_mall_phantom.c: In function ‘set_phantom_stream_timing’:
drivers/gpu/drm/amd/amdgpu/../display/dc/dml2/dml2_mall_phantom.c:657:25: warning: variable ‘pipe_idx’ set but not used [-Wunused-but-set-variable=]
  657 |         unsigned int i, pipe_idx;
      |                         ^~~~~~~~

Signed-off-by: Brahmajit Das <listout@listout.xyz>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>