drm/amdgpu: Add a new flag for SR-IOV to share memory between PF & VF

Add ATOM_VRAM_BLOCK_SRIOV_MSG_SHARE_RESERVATION to identify whether
driver need to reserve VRAM for SR-IOV shared memory.

Signed-off-by: Horace Chen <horace.chen@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>