drm/amd/display: fix 4K stereo screen flash issue

[Why]
HDMI_scramber is not enabled for pixel rate >340Mhz.
[How]
Calculate the phy clock to include the Hw frame packing factor.

Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Chris Park <Chris.Park@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>