drm/xe: Consolidate locking of vm and external objects dma-resv

The locking of the vm's dma-resv and external objects' dma_resvs were open-coded at least at three different locations. Replace those with a helper function and also remove the struct ttm_validate_buffers embedded in the buffer objects since multiple users having the vm lock in read mode could race to use those. Instead allocate these either on stack or using kvmalloc() when needed only.

Signed-off-by: Thomas Hellström thomas.hellstrom@linux.intel.com