drm/i915/selftest: Update the SLPC selftest

Use the new efficient frequency toggling interface. Also
create a helper function to restore the frequencies after
the test is done.

v2: Restore max freq first and then min.

Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230426003942.1924347-2-vinay.belgaumkar@intel.com