drm/i915: add encoder->pre_pll_enable callback

Currently we have two encoder specific bits in the common mode_set
functions:
- lvds pin pair enabling
- dp m/n setting and computation

Now the lvds stuff needs to happen before the pll is enabled. Since
that is done in the crtc_mode_set functions, we need to add a new
callback to be able to move them to the encoder code (where they
belong). The dp m/n stuff is a giant mess anyway (since it also
confuses itself with the fdi link m/n handling), so that needs to be
handled separately.

I think that we can move the pll enabling down quite a bit, which
might allow us to eventually merge encoder->pre_enable with this new
pre_pll_enable callback. But for now this will allow us to clean
things up a bit.

Note that vlv doesn't support lvds, hence we don't need to change
anything in there.

v2: Fixup commit message, both suggested from Paulo Zanoni.
- dp m/n doesn't need to happen before pll enabling
- lvds doesn't exist on vlv, hence no changes required in the vlv pll
  function.

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