ice: Slightly simply ice_find_free_recp_res_idx

The 'possible_idx' bitmap is set just after it is zeroed, so we can save
the first step.

The 'free_idx' bitmap is used only at the end of the function as the
result of a bitmap xor operation. So there is no need to explicitly
zero it before.

So, slightly simply the code and remove 2 useless 'bitmap_zero()' call

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Tested-by: Sandeep Penigalapati <sandeep.penigalapati@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>