drm/i915: move find_pll callback to dev_priv->display

drm/i915: move find_pll callback to dev_priv->display

Now that the DP madness is cleared out, this is all only per-platform.
So move it out from the intel clock limits structure.

While at it drop the intel prefix on the static functions, call the
vtable entry find_dpll (since it's for the display pll) and rip out
the now unnecessary forward declarations.

Note that the parameters of ->find_dpll are still unchanged, but they
eventually need to be moved over to just take in a pipe configuration.
But currently a lot of things are still missing from the pipe
configuration (reflock, output-specific dpll limits and preferences,
downclocked dotclock). So this will happen in a later step.

Note that intel_g4x_limit has a peculiar case where it selects
intel_limits_i9xx_sdvo as the limit. This is pretty bogus and also not
used since the only output types left are DP and native TV-out which
both use special pre-tuned dpll values.

v2: Re-add comment for the find_pll callback (requested by Paulo) and
elaborate on why the transformation is correct for g4x platforms (to
clarify a review question from Paulo). Double up on that by adding a
WARN as suggested by Paulo Zanoni on irc.

v3: Initialize limits to NULL since gcc is now unhappy.

v4: v2/3 will blow up with a NULL dereference in ->find_dpll for dp and
TV-out ports, spotted by Paulo on irc. So just give up on this madness for
now, and leave this to be fixed in a later patch.

v5: Since the ever-so-slight change for g4x might result in some dpll
parameter computation failing spuriously where before it didn't for
ports with preset dpll settings (DP & TV-out) override this. For
paranoia also do it in the ilk+ code.

Cc: Paulo Zanoni <przanoni@gmail.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>