thermal: intel_powerclamp: Use bitmap_zalloc/bitmap_free when applicable

'cpu_clamping_mask' is a bitmap. So use 'bitmap_zalloc()' and
'bitmap_free()' to simplify code, improve the semantic of the code and
avoid some open-coded arithmetic in allocator arguments.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>