drm/imagination: Fix ERR_PTR test on pointer to pointer.

drivers/gpu/drm/imagination/pvr_vm.c:631 pvr_vm_create_context()
  error: 'vm_ctx->mmu_ctx' dereferencing possible ERR_PTR()

612         vm_ctx->mmu_ctx = pvr_mmu_context_create(pvr_dev);
613         err = PTR_ERR_OR_ZERO(&vm_ctx->mmu_ctx);
                                      ^
The address is never an error pointer so this will always return 0.
Remove the ampersand.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Donald Robson <donald.robson@imgtec.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231213144431.94956-1-donald.robson@imgtec.com