drm/i915/gsc: take a wakeref for the proxy-init-completion check

Ensure intel_gsc_uc_fw_init_done and intel_gsc_uc_fw_proxy_init
takes a wakeref before reading GSC Shim registers.

NOTE: another patch in review also adds a call from selftest to
this same function. (https://patchwork.freedesktop.org/series/117713/)
which is why i am adding the wakeref inside the callee, not the
caller.

v2: - add a helper, 'gsc_uc_get_fw_status' for both callers
      (Daniele Ceraolo)

Fixes: 99afb7cc

 ("drm/i915/pxp: Add ARB session creation and cleanup")
Signed-off-by: Alan Previn <alan.previn.teres.alexis@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230608230716.3079594-1-alan.previn.teres.alexis@intel.com