drm/atomic: Initialise planes with opaque alpha values

Planes without an alpha property, using __drm_atomic_helper_plane_reset
will have their plane state alpha initialised as zero, which represents
a transparent alpha.

If this value is then used for the plane, it may not be visible by
default, and thus doesn't represent a good initialisation state.

Update the default state->alpha value to DRM_BLEND_ALPHA_OPAQUE
unconditionally when the plane is reset.

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180919155700.10342-2-kieran.bingham+renesas@ideasonboard.com