drm/amd/display: Respect CONFIG_FRAME_WARN=0 in DML2

display_mode_code.c is unconditionally built with
-Wframe-larger-than=2048, which causes warnings even when
CONFIG_FRAME_WARN has been set to 0, which should show no warnings.

Use the existing $(frame_warn_flag) variable, which handles this
situation. This is basically commit 25f178bb ("drm/amd/display:
Respect CONFIG_FRAME_WARN=0 in dml Makefile") but for DML2.

Fixes: 7966f319 ("drm/amd/display: Introduce DML2")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>