drm/meson: The old 32-bit SoCs don't have the OSD alpha replace bits

The VIU_OSD1_CTRL_STAT2 and VIU_OSD2_CTRL_STAT2 registers on
Meson8/Meson8b/Meson8m2 don't have the following bits:
- replaced_alpha_en in bit [14]
- replaced_alpha in bits [13:6]

Add checks to the driver to skip setting those for these older SoCs.

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