ACPI: thermal: Rework thermal_get_trend()

Rework the ACPI thermal driver's .get_trend() callback function,
thermal_get_trend(), so that it does not call thermal_get_trip_type()
and thermal_get_trip_temp() which are going to be dropped.

This reduces the overhead of the function too, because it will always
carry out a trip point lookup once after the change.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>