drm/amd/display: Disable display writeback on Linux for NV10

[WHY] system crash when initialize dwb
current linux driver does not support dwb.
 disable this feature for now.

[HOW] set num_dwb = 0 to disable dwb for now

Signed-off-by: hersen wu <hersenxs.wu@amd.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>