drm/i915/wm: provide wrappers around watermark vfuncs calls (v3)

This moves one wrapper from the pm->display side, and creates
wrappers for all the others, this should simplify things later.

One thing to note is that the code checks the existence of some
of these ptrs, so the wrappers are a bit complicated by that.

Suggested by Jani.

v2: fixup warnings in wrong place error.

v3 by Jani: fix intel_compute_global_watermarks() return value check

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ee2760c45896568c9dd9114a575509619bd44ef2.1632869550.git.jani.nikula@intel.com