drm/amd/display: move bpp range decision in decide dsc bw range function

[why]
Before get dsc bw range is used to compute DSC bw range
based on the given fixed bpp min/max input.
The new change will merge any specs, signal, timing specific
bpp range decision into this function. So the function needs to make
a decision with all aspects considered.

Acked-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Reviewed-by: George Shen <george.shen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>