drm/meson: Fix an Alpha Primary Plane bug on Meson GXL/GXM SoCs

On the Amlogic GXL & GXM SoCs, a bug occurs on the primary plane when
alpha is used where the alpha is not aligned with the pixel content.

The workaround Amlogic implemented is to reset the OSD1 plane hardware
block each time the plane is (re)enabled, solving the issue.

In the reset, we still need to save the content of 2 registers which
depends on the status of the plane, in addition to reload the scaler
conversion matrix at the same time.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Maxime Jourdan <mjourdan@baylibre.com>
Reviewed-by: Maxime Jourdan <mjourdan@baylibre.com>
[narmstrong: fixed typo in commit log]
Link: https://patchwork.freedesktop.org/patch/msgid/20181128100734.6536-1-narmstrong@baylibre.com