tests/amdgpu/amd_dp_dsc: fix basic and bpc test be skipped

amd_dp_dsc tests are skipped when output max value (n_outputs)
is fewer than pipe max value (n_pipes) because a pipe may not
be enabled, i.e. display->pipes[i].enable == false. As a result,
n_outputs should be used instead.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>