drm/amd/display: Correct prefetch calculation

[Why]
The minimum value of the dst_y_prefetch_equ was not correct
in prefetch calculation whice causes OPTC underflow.

[How]
Add the min operation of dst_y_prefetch_equ in prefetch calculation
for legacy DML.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: TungYu Lu <tungyu.lu@amd.com>
Signed-off-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Tested-by: Mark Broadworth <mark.broadworth@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>