drm/amd/display: Add reinstate dram in the FPO logic

In order to handle FPO correctly, we need to reinstate the dram values.
This function adds the required code to handle the vblank stretch and
the dram calculation.

Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>