accel/ivpu: Allocate vpu_addr in gem->open() callback

Use gem->open() callback to simplify the code and prepare for gem_shmem
conversion. It is called during handle creation for a gem object,
during prime import and in BO_CREATE ioctl. Hence can be used for vpu_addr
allocation. On the way remove unused bo->user_ptr field.

Signed-off-by: Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231031073156.1301669-2-stanislaw.gruszka@linux.intel.com