drm/amd/display: Avoid setting pixel rate divider to N/A

[Why]
Pixel rate divider values should never be set to N/A (0xF) as the K1/K2
field is only 1/2 bits wide.

[How]
Set valid divider values for virtual and FRL/DP2 cases.

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Taimur Hassan <Syed.Hassan@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>