drm/amd/display: Replace repeated no-native-i2c checks with force_i2c_over_aux field

drm/amd/display: Replace repeated no-native-i2c checks with force_i2c_over_aux field

[Why]
The compound condition checking dp_connector_no_native_i2c
and no_ddc_pin was duplicated across many files, obscuring
intent at every call site.

[How]
Add bool force_i2c_over_aux to struct dc_link, initialized
once during link creation. Add link_get_ddc_aux_inst()
helper to select the correct aux instance. Wire into
link_service via construct_link_service_ddc(). Replace all
duplicated condition checks and aux instance selection
blocks with the new field and helper. No functional change.

Reviewed-by: Nevenko Stupar <nevenko.stupar@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: George Zhang <george.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>