drm/amd/display: Refactor input mode programming for DIG FIFO

[WHY]
Input mode for the DIG FIFO should be programmed as part of stream
encoder setup.

[HOW]
Pre-calculate the pixels per cycle as part of the pixel clock params,
and program as part of stream encoder setup.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Dillon Varone <dillon.varone@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>