Update the UBWC registers to the right values for sm8150 and sm8250.

This removes broken dpu_hw_reset_ubwc, which doesn't work because the
"force blk offset to zero to access beginning of register region" hack is
copied from downstream, where mapped region starts 0x1000 below what is
used in the upstream driver.

Also simplifies the overly complicated change that was introduced in
e4f9bbe9 to work around dpu_hw_reset_ubwc being broken.
Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Signed-off-by: Rob Clark <robdclark@chromium.org>