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.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Lo-an Chen <lo-an.chen@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>