drm/amd/display: HP Reverb G2 VR fails to light up

[Why]
Many VR headsets require a HSYNC width of 4, but DCN
has default minimum of 8.

[How]
Change the arbitrary minimum HSYNC width to 4 to match
DCN20.

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>