drm/exynos: mixer: set window priority based on zpos

'zpos' plane property is configurable, so adjust hardware layers
priority based on the zpos value. 'zpos' value shifted by one can be
used directly as hw priority value and stored to the registers, because
mixer accepts priority values from 1 to 15 (0 means that layer is
disabled).

This patch also changes the default layer priority to match already
exposed initial zpos values. The initial configuration is now:
[top] video > gfx layer1 > gfx layer0 [bottom].

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>