drm/i915: Clarify sprite plane function names (v4)

A few of the sprite-related function names in i915 are very similar
(e.g., intel_enable_planes() vs intel_crtc_enable_planes()) and don't
make it clear whether they only operate on sprite planes, or whether
they also apply to all universal plane types.  Rename a few functions to
be more consistent with our function naming for primary/cursor planes or
to clarify that they apply specifically to sprite planes:

 - s/intel_disable_planes/intel_disable_sprite_planes/
 - s/intel_enable_planes/intel_enable_sprite_planes/

Also, drop the sprite-specific intel_destroy_plane() and just use
the type-agnostic intel_plane_destroy() function.  The extra 'disable'
call that intel_destroy_plane() did is unnecessary since the plane will
already be disabled due to framebuffer destruction by the point it gets
called.

v2: Earlier consolidation patches have reduced the number of functions
    we need to rename here.

v3: Also rename intel_plane_funcs vtable to intel_sprite_plane_funcs
    for consistency with primary/cursor.  (Ander)

v4: Convert comment for intel_plane_destroy() to kerneldoc now that it
    is no longer a static function.  (Ander)

Reviewed-by(v1): Bob Paauwe <bob.j.paauwe@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>