drm/amd/display: Don't set 4to1MPC config dynamically

drm/amd/display: Don't set 4to1MPC config dynamically

We were previously modifying the global dc->config.enable_4to1MPC
dynamically. These variables are meant as global configs, not to
by dynamically modified. Modifying them dynamically prevents us
from enabling/disabling functionality for debug purposes and can
easily lead to bad things since we're not operating on the current
state but on DC-wide variables.

Instead we should look at the existing split4mpc decision in
dcn20_validate_apply_split_flags and make the decision there,
if the global config.enable_4to1MPC is set to true for the
DCN version we're running.

This fixes corruption that is observed when running a new IGT
kms_colorop test for color-space-conversion that uses a
YUV plane and outputs to a writeback connector.

Co-developed by Claude Sonnet 4.5.

Assisted-by: Claude:claude-sonnet-4.5
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Chuanyu Tseng <chuanyu.tseng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>