drm/amdgpu/vcn: Use buffer object's deletion logic

VCN DPG buffer object is intialized to NULL. If allotted, buffer object
deletion logic will take care of NULL check and delete accordingly. This
is useful for cases where indirect sram flag could be manipulated later
after buffer allocation.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>