drm/amd/display: Remove redundant var from display_rq_dig_calc in dml

drm/amd/display: Remove redundant var from display_rq_dig_calc in dml

[WHY]
Coverity analysis flagged these files as containing DEADCODE, i. e.
redundant or unreachabale. In these files, variable 'mode_422' is
always the same file (0 or false), so any code logic with it is
redundant.

[HOW]
Removed 'mode_422' variable and all code with it from
'display_rq_dlg_calc_' functions in dml.

Reviewed-by: Alex Hung <[email protected]>
Signed-off-by: Alex Hung <[email protected]>
Signed-off-by: Ivan Lipski <[email protected]>
Tested-by: Daniel Wheeler <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>