drm/i915/dsi: Remove ->mode_set callback

Looking at our current dsi driver I note that:
- We don't have any slave driver right now.
- There's zero support for the hardware state readout and cross check
  code.
- All the modeset state seems to be tracked in the intel_dsi structure
  instead of the pipe config.

Given all that I can't properly audit the dsi ->mode_set callback. So
just do it as the first thing in the ->pre_pll_enable hook and hope
for the best.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>