drm/gpuvm: Kill drm_gpuva_init()

drm_gpuva_init() only has one internal user, and given we are about to
add new optional fields, it only add maintenance burden for no real
benefit, so let's kill the thing now.

Cc: Danilo Krummrich <dakr@kernel.org>
Cc: Rob Clark <robin.clark@oss.qualcomm.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Reviewed-by: Rob Clark <robin.clark@oss.qualcomm.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250819162058.2777306-3-himal.prasad.ghimiray@intel.com