media: s5p-mfc: Use 'bitmap_zalloc()' when applicable

'mfc_dev->mem_bitmap' is a bitmap. So use 'bitmap_zalloc()' to simplify
code and improve the semantic.

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

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>