drm/amd/display: fix k1 k2 divider programming for phantom streams

[Why & How]
When k1 and k2 divider programming logic is executed for a phantom
stream, the corresponding master stream should be used for the
calculation. Fix the if condition to use the master stream for checking
signal type instead of the phantom stream.

Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>