drm/i915/display: Rename POWER_DOMAIN_DPLL_DC_OFF to POWER_DOMAIN_DC_OFF

This power domain to disable DC states will be used in places outside
of DPLL, so making the name more generic.

Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Caz Yokoyama <caz.yokoyama@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211020003558.222198-1-jose.souza@intel.com