drm/gpuvm: fix missing dependency to DRM_EXEC

In commit 50c1a36f ("drm/gpuvm: track/lock/validate external/evicted
objects") we started using drm_exec, but did not select DRM_EXEC in the
Kconfig for DRM_GPUVM, fix this.

Cc: Christian König <christian.koenig@amd.com>
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Fixes: 50c1a36f ("drm/gpuvm: track/lock/validate external/evicted objects")
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240715135158.133287-1-dakr@redhat.com