drm/i915: Shovel hw setup code out of i9xx_crtc_mode_set

All these functions simply convert sw state as encoded in the pipe
config or primary framebuffer into hardware state. So we can move them
all into the crtc enable hook. Unfortunately this means a little bit
of duplication between the i9xx and vlv functions, but since we
already have highly refactored code I think this is acceptable.

Also a pile of forward declarations unfortunately.

Note also that the various <platform>_update_pll functions are still
called from within the ->crtc_mode_set hook. Mostly they compute the
clock state for the pipe config, but unfortunately there are some
random register writes interspersed. Those need to be moved out first
before we can enable runtime PM for DPMS.

Reviewed-by: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>