drm/meson: update meson_vpu_init to work with Meson8/Meson8b/Meson8m2

Don't modify the VPU_RDARB_MODE_* registers because they only exist on
GXBB and newer SoCs. Initialize the VPU_MEM_PD_REG0 and VPU_MEM_PD_REG1
to 0x0 (meaning: enable everything), just like vendor u-boot does for
Meson8/Meson8b/Meson8m2.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>