drm/i915: consolidate pch pll enable sequence

drm/i915: consolidate pch pll enable sequence

It's been splattered over 3 different places all doing random things.
Now we have (mostly) the same sequence as i8xx/i9xx, but all called
from the crtc_enable hook (through the pll->enable function):
- write new dividers
- enable vco and wait for stable clocks
- write again for the pixel mutliplier

I've left the seemingly random 200 usec delay in there, just in case.

Also move the encoder->pre_pll_enable hook into the crtc_enable
function, at the same spot we currently have a hack to enable the lvds
port. Since that hack is now redundant, kill it.

While doing this patch I've learned the hard way that we can only fire
up the LVDS port if both the pch dpll _and_ the fdi rc pll are not yet
enabled. Otherwise things go haywire, at least on cpt.

v2: It is paramount to write the FPx divisors before we enable the
the vco by writing to the DPLL registers, for otherwise the divisors
won't get updated. This is in line with the i8xx/i9xx dpll.

v3: To keep the nice abstraction add a ->mode_set callback to set the
divisors. Also streamline the enabling/disabling code a bit by
removing some cargo-cult duplication and clearing registers where
possible in the ->disable hook.

v4: Remove now unused local variable.

Acked-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>