drm/amd/display: remove DML Makefile duplicate lines

There are two identical CFLAGS entries for "display_mode_vba_20.o", so
remove one of them. Also, as there's already an entry for
"display_mode_lib.o" CFLAGS, regardless of CONFIG_DRM_AMD_DC_DCN being
defined or not, remove the one entry between CONFIG_DRM_AMD_DC_DCN ifdef
guards.

Signed-off-by: Magali Lemes <magalilemes00@gmail.com>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>