drm/amd/display: Don't disable dtb as dto src during dpms off

[WHY & HOW]
Adjusting otg dto programming in this path is unsafe otg may be en at
this time, swapping its src sel for example was discovered to cause
visual artifacts and was moved to dcn31_program_pix_clk (where otg is
guaranteed off) but dto functional clk enable bit is not cleared.

dcn31_program_pix_clk is called during dpms on, so until that point
we will have a otg with functional src sel set (Sel DTBclk) and that
functional clk dto en is unset (i.e DTB DTO EN), which is invalid and tg
will not function and system will hang on something such as flip
incoming from OS during dpms off. will scope out dcn315 as it was
affecting diags tests on certain CI machines (not crbs)

Reviewed-by: Leo Chen <leo.chen@amd.com>
Signed-off-by: Ausef Yousof <Ausef.Yousof@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>