drm/amd/display: Update underflow detection for DCN42

drm/amd/display: Update underflow detection for DCN42

[Why]
The DCN42 underflow detection functions in dcn42_optc.c use
OPTC_RSMU_UNDERFLOW register but the register offset definitions
were missing from dcn_4_2_0_offset.h and dcn42_resource.h.

[How]
Add missing register definitions.

Fixes: e56e3cff2a1b ("drm/amd/display: Sync dcn42 with DC 3.2.373")
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Roman Li <Roman.Li@amd.com>
Signed-off-by: Chuanyu Tseng <Chuanyu.Tseng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>