imx-drm plane, build warning, and error handling fixes

- some fixes for active plane reconfiguration support
- hide unused label in case of disabled CONFIG_DRM_FBDEV_EMULATION,
  which caused a build warning
- fixed error handling in imx_drm_bind
- disallow odd x/y plane offsets for chroma subsampled formats
- disable local alpha when switching from a format with alpha
  channel to an opaque format
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