drm/amd/display: fix workaround for incorrect double buffer register for DLG ADL and TTU

[Why]
these registers should have been double buffered. SW workaround we will have SW program the more aggressive (lower) values
whenever we are upating this register, so we will not have underflow at expense of less optimzal request pattern.

[How]
there is a driver bug where we don't check for 0, which is uninitialzed HW default.  since 0 is smaller than any value we need to program,
driver end up with not programming these registers

Signed-off-by: Tony Cheng <tony.cheng@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>