drm/xe/dsb: DSB implementation for xe

Add xe specific DSB buffer handling methods.

v1: Initial version.
v2: Add null check after dynamic memory allocation of vma. [Uma]

Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>