drm/i915/wm: move remaining watermark code out of intel_pm.c

Add new files intel_wm.[ch] and i9xx_wm.[ch] under display/ to hold
generic and pre-SKL watermark code, respectively. SKL+ watermark code
has already been split out to skl_watermark.[ch].

Use the _wm.[ch] naming for brevity; we may want to rename
skl_watermark.[ch] later accordingly.

Add new intel_wm_init() to call either skl_wm_init() or
i9xx_wm_init(i915) depending on the platform, the latter comprising of
the remains of intel_init_pm().

Sprinkle in some minor checkpatch fixes while moving the code.

v2:
- Rebase
- Fix copyright year

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/ddf04a07a37f0368b3fef85d4ebb924082fec6cd.1676317696.git.jani.nikula@intel.com