drm/imagination: Removed unused function to_pvr_vm_gpuva()

drm/imagination: Removed unused function to_pvr_vm_gpuva()

This function is now unused, hence it causes a compiler warning.

   drivers/gpu/drm/imagination/pvr_vm.c:112:22: warning: unused function 'to_pvr_vm_gpuva' [-Wunused-function]
     112 | struct pvr_vm_gpuva *to_pvr_vm_gpuva(struct drm_gpuva *gpuva)
         |                      ^

Remove the function for now.

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>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231130160017.259902-5-donald.robson@imgtec.com