imx-drm module/dependency changes

- The PRE/PRG drivers added an unwanted DRM dependency to the ipu-v3 driver.
  Remove the dependency by conditionally disabling PRE/PRG support depending
  on CONFIG_DRM.
- Merge the imx-ipuv3-crtc module into the imxdrm module. There is no reason
  anymore for a separation between core drm driver and crtc/plane drivers,
  especially since commit eb8c88808c83 ("drm/imx: add deferred plane
  disabling"), which added a dependency on imx-ipuv3-crtc to the imxdrm
  module.
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