drm/i915/display: Be explicit in handling the preallocated vma

As only the display codes tries to pin its preallocated framebuffer into
an exact location in the GGTT, remove the convenience function and make
the pin management explicit in the display code. Then throughout the
display management, we track the framebuffer and its plane->vma; with
less single purpose code and ready for first class i915_vma.

In doing so, this should fix the BUG_ON(vma->pages) on fi-kbl-soraka.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200204094801.877288-2-chris@chris-wilson.co.uk