drm/amd/display: kernel-doc: document force_timing_sync

As warned when running "make htmldocs":

	./drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h:345: warning: Function parameter or member 'force_timing_sync' not described in 'amdgpu_display_manager'

This new struct member was not documented at kernel-doc markup.

Fixes: 3d4e52d0 ("drm/amd/display: Add debugfs for forcing stream timing sync")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>