drm/amd/display: remove unused variable available

With gcc and W=1, there is this error
drivers/gpu/drm/amd/amdgpu/../display/dc/link/protocols/link_dp_dpia_bw.c:297:13: error:
  variable ‘available’ set but not used [-Werror=unused-but-set-variable]
  297 |         int available = 0;
      |             ^~~~~~~~~

Since available is unused, remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>