drm/amd/display: fix asserts in SPL during bootup

[Why]
During mode validation, there maybe modes that fail
 max_downscale_src_width check and scaling_quality
 taps are 0.  This will cause an assert to trigger
 in spl_set_filters_data() because taps are 0.

[How]
Move taps calculation for non-adaptive scaling mode
 to separate function and call it
 if max_downscale_src_width fails.  This will
 populate taps if scaling_quality taps are 0.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Samson Tam <Samson.Tam@amd.com>
Signed-off-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>