drm/i915/dsm: remove unnecessary dsm priv structure

Pass a local acpi_handle around instead of having a static dsm priv
structure. If we need it later, we can always move it to dev_priv, and
the change at hand will make that easier as well.

Care is taken to preserve old behaviour, particularly using the last
non-NULL acpi handle, whether it makes sense or not.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180614104709.2808-1-jani.nikula@intel.com