drm/i915/pxp: fix undefined reference to `intel_pxp_gsccs_is_ready_for_sessions'

drm/i915/pxp: fix undefined reference to `intel_pxp_gsccs_is_ready_for_sessions'

On x86_64 with gcc version 13.3.0, I compile kernel with:

  make defconfig
  ./scripts/kconfig/merge_config.sh .config <(
    echo CONFIG_COMPILE_TEST=y
  )
  make KCFLAGS="-fno-inline-functions -fno-inline-small-functions -fno-inline-functions-called-once"

Then I get a linker error:

  ld: vmlinux.o: in function `pxp_fw_dependencies_completed':
  kintel_pxp.c:(.text+0x95728f): undefined reference to `intel_pxp_gsccs_is_ready_for_sessions'

This is caused by not having a intel_pxp_gsccs_is_ready_for_sessions()
header stub for CONFIG_DRM_I915_PXP=n. Add it.

Signed-off-by: Chen Linxuan <chenlinxuan@uniontech.com>
Fixes: 99afb7cc8c44 ("drm/i915/pxp: Add ARB session creation and cleanup")
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://lore.kernel.org/r/20250415090616.2649889-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>