drm/i915/dmc: remove unnecessary dmc_id validity check

The dmc_id comes from for_each_dmc_id() in parse_dmc_fw() ->
parse_dmc_fw_header() -> dmc_mmio_addr_sanity_check(). It's valid by
definition.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b4e1c862687f79861a5fa4bfa6797ecda2136fab.1675339447.git.jani.nikula@intel.com