drm/amd/display: add public taps API in SPL

[Why]
Add public API to obtain number of taps in SPL.

[How]
Isolate function to calculate recout, ratios and viewport before
calculating taps. Call function in both public taps API call and private
scaling call.

Reviewed-by: Jun Lei <jun.lei@amd.com>
Signed-off-by: Samson Tam <Samson.Tam@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>