drm/amdkfd: Use bitmap_zalloc() when applicable

'doorbell_bitmap' and 'queue_slot_bitmap' are bitmaps. So use
'bitmap_zalloc()' to simplify code, improve the semantic and avoid some
open-coded arithmetic in allocator arguments.

Also change the corresponding 'kfree()' into 'bitmap_free()' to keep
consistency.

Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>