drm/amd/display: Use double buffered DRR timing update by default

[Why]
For some monitors extreme flickering can occur while using LFC for if
we're not doing the DRR timing update for V_TOTAL_MIN / V_TOTAL_MAX at
the DP start of frame.

Hardware can default to any time in the frame which isn't the behavior
we want.

[How]
Add a new function for setting the double buffering mode for DRR timing.

Default to DP start of frame double buffering on timing generator init.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>