MAINTAINERS: entry for DRM GPUVM

GPUVM deserves a bit more coordination, also given the upcoming Rust
work for GPUVM, hence add a dedicated maintainers entry for DRM GPUVM.

Cc: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Acked-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Matthew Brost <matthew.brost@intel.com>
Acked-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250808092432.461250-1-dakr@kernel.org


Signed-off-by: Danilo Krummrich <dakr@kernel.org>