drm/i915/icl: Unify disable and enable phy clock gating functions

Adding a enable parameters allow us to share most of the code between
enable and disable functions.

v3:
Renamed icl_phy_clock_gating() to icl_phy_set_clock_gating()

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190920205810.211048-6-jose.souza@intel.com