drm/i915/opregion: abstract ASLE presence check

Add a function to check the opregion ASLE presence instead of accessing
the opregion structures directly.

Reorder the checks in i915_has_asle() to avoid the function call if
possible.

Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1829415d3e7b29e78c46b20bca8175ef373bcd92.1704992868.git.jani.nikula@intel.com