drm: Unexport primary plane helpers

Well except the destroy helper, which isn't really a primary helper
but generally useful, if mislabelled.

v2: Keep some of the nice comments about the limitations of the
primarmy plane helpers, and put them into the kerneldoc for
drm_crtc_init() (Sam).

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181005094732.31353-1-daniel.vetter@ffwll.ch