drm/i915: Use plane->state->fb in watermark code (v2)

plane->fb is a legacy pointer that not always be up-to-date (or updated
early enough).  Make sure the watermark code uses plane->state->fb so
that we're always doing our calculations based on the correct
framebuffers.

This patch was generated by Coccinelle with the following semantic
patch:

        @@
        struct drm_plane *P;
        @@
        - P->fb
        + P->state->fb

v2: Rebase

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>