drm/amd/display: Add a missing hook to DCN20

The struct timing_generator_funcs provides a hook for setting up the
maximum possible vertical dimension of display for OTG, as the panel
supports. DCN10 has a standard function named optc1_set_vtotal_min_max
which all ASICs can use to set the aforementioned hook. Since we did not
set it for DCN20, this commit initializes the set_vtotal_min_max with
the DCN10 function.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>