drm/imagination: pvr_gpuvm_free() now static

drm/imagination: pvr_gpuvm_free() now static

The function below is used only within this source file, but is not static.

drivers/gpu/drm/imagination/pvr_vm.c:542:6: error: no previous prototype for 'pvr_gpuvm_free' [-Werror=missing-prototypes]
  542 | void pvr_gpuvm_free(struct drm_gpuvm *gpuvm)

Make it static.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202311242159.hh8MWiAm-lkp@intel.com/
Fixes: ff5f643de0bf ("drm/imagination: Add GEM and VM related code")
Signed-off-by: Donald Robson <donald.robson@imgtec.com>
Link: https://lore.kernel.org/r/20231130160017.259902-4-donald.robson@imgtec.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>