drm/i915/wm: add .get_hw_state to watermark funcs

Get rid of the if ladder in intel_modeset_setup_hw_state() and hide a
number of functions by adding a .get_hw_state() hook to watermark
functions. At least for now, combine the platform specific sanitization
to the hw state readouts on the relevant platforms instead of adding a
separate hook for that.

There's a functional change on PCH split platforms: If i9xx_wm_init()
fails to read plane latency and chooses the nop functions,
ilk_wm_get_hw_state() won't get called for readout. Add the
ilk_init_lp_watermarks() call on that path which now won't be called in
.get_hw_state(), as it looks like the only thing that could make a
difference.

v2:
- Add missing static (kernel test robot)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6da32831e40606cc8b90491b83196917f2ce36ab.1676317696.git.jani.nikula@intel.com