drm/amd/display: fix calculation of total_data_read_bandwidth

[why]
by adding fast_validate flag, we are skipping some portion of
dcn_validate_bandwidth code that is not necessary for mode validation.
However we have a bug where it does not calculate
v->total_data_read_bandwidth, which is one of the factors determines the
result of the validation, and therefore report false positive during
mode validation.

[how]
add calculation of v->total_data_read_bandwidth outside of the region
that is guarded by fast_validate flag

Signed-off-by: Su Sung Chung <Su.Chung@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>