drm/i915: Calculate ILK-style watermarks during atomic check (v3)

Calculate pipe watermarks during atomic calculation phase, based on the
contents of the atomic transaction's state structure.  We still program
the watermarks at the same time we did before, but the computation now
happens much earlier.

While this patch isn't too exciting by itself, it paves the way for
future patches.  The eventual goal (which will be realized in future
patches in this series) is to calculate multiple sets up watermark
values up front, and then program them at different times (pre- vs
post-vblank) on the platforms that need a two-step watermark update.

While we're at it, s/intel_compute_pipe_wm/ilk_compute_pipe_wm/ since
this function only applies to ILK-style watermarks and we have a
completely different function for SKL-style watermarks.

Note that the original code had a memcmp() in ilk_update_wm() to avoid
calling ilk_program_watermarks() if the watermarks hadn't changed.  This
memcmp vanishes here, which means we may do some unnecessary result
generation and merging in cases where watermarks didn't change, but the
lower-level function ilk_write_wm_values already makes sure that we
don't actually try to program the watermark registers again.

v2: Squash a few commits from the original series together; no longer
    leave pre-calculated wm's in a separate temporary structure since
    it's easier to follow the logic if we just cut over to using the
    pre-calculated values directly.

v3:
 - Pass intel_crtc instead of drm_crtc to .compute_pipe_wm() entrypoint
   and use intel_atomic_get_crtc_state() to avoid need for extra
   casting.  (Ander)
 - Drop unused intel_check_crtc() function prototype.  (Ander)

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Smoke-tested-by: Paulo Zanoni <przanoni@gmail.com>
Link: http://patchwork.freedesktop.org/patch/60363/