drm/amd/display: handle max_downscale_src_width fail check

[WHY]
If max_downscale_src_width check fails, we exit early from TAP calculation and left a NULL
value to the scaling data structure to cause the zero divide in the DML validation.

[HOW]
Call set default TAP calculation before early exit in get_optimal_number_of_taps due to
max downscale limit exceed.

Reviewed-by: Samson Tam <samson.tam@amd.com>
Signed-off-by: Yihan Zhu <Yihan.Zhu@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>