drm: Add drm_crtc_init_with_planes() (v2)

Add a new drm_crtc_init_with_planes() to allow drivers to provide
specific primary and cursor planes at CRTC initialization.  The existing
drm_crtc_init() interface remains to avoid driver churn in existing
drivers; it will initialize the CRTC with a plane helper-created primary
plane and no cursor plane.

v2:
  - Move drm_crtc_init() to plane helper file so that nothing in the DRM
    core depends on helpers.  [suggested by Daniel Vetter]
  - Keep cursor parameter to drm_crtc_init_with_planes() a void* until
    we actually add cursor support.  [suggested by Daniel Vetter]

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>