drm/amd/display: Remove min_dst_y_next_start check for Z8

[Why]
Flickering occurs on DRR supported panels when engaged in DRR due to
min_dst_y_next becoming larger than the frame size itself.

[How]
In general, we should be able to enter Z8 when this is engaged but it
might be a net power loss even if the calculation wasn't bugged.

Don't support enabling Z8 during the DRR region.

Cc: stable@vger.kernel.org # 6.1+
Reviewed-by: Syed Hassan <syed.hassan@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>