drm/amd/display: Remove always-false branches

[WHAT & HOW]
MacroTileSizeBytes is set to either 256 or 65535 and it is never
4096. Its branch is not taken, and should be removed. Similarly,
mode_422 is always 0 and thus ppe will always be 1. The ternary
operator should be removed.

This fixes 2 DEADCODE issues reported by Coverity.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Fangzhi Zuo <jerry.zuo@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>