drm/i915/guc: define media GT GuC send regs

The media GT shares the G-unit with the root GT, so a second set of
communication registers is required for the media GuC.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Alan Previn <alan.previn.teres.alexis@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221108020600.3575467-6-daniele.ceraolospurio@intel.com