drm/i915/dp: convert to encoder disable/enable

DP is the first encoder which isn't simple. As

commit d240f20f
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Fri Aug 13 15:43:26 2010 -0700

    drm/i915: make sure eDP PLL is enabled at the right time

discovered, we need to enable the eDP PLL for the cpu port _before_ we
enable the pipes and planes. After a few more commits the current
solution is to enable the PLL in the dp mode_set function (because
this is the only encoder callback the crtc helper code calls before it
calls the crtc's commit function).

Now I suspect that we actually should enable/disable the entire cpu
eDP port before/after planes, but thanks to how the crtc helper code
assumes that you can disable an encoder without disabling it's crtc
right away, this won't work.

The result is that the current prepare/commit hooks don't touch the
eDP PLL, but instead it get's frobbed in dp_mode_set and in the dp
dpms function. Hence we need to keep things (at least for now)
bug-for-bug compatible by using our own special dp dpms function and
keep everything else more-or-less as-is (just using our own
infrastrucutre now).

This mess can only be cleaned up once we control the entire modeset
sequence and can move things around freely.

v2: Squash unsupported dpms modes to OFF at the beginning of the DP
dpms function.

v3: Need to set the dpms state to off in dp_disable, otherwise this
breaks the newly added WARNs ...

v4: Rebased against edp panel off sequence changes in 3.6-rc2

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>