[media] s5p-tv: mixer: Prepare for common clock framework

Replace clk_enable() by clock_enable_prepare() and clk_disable()
with clk_disable_unprepare(). clk_{prepare/unprepare} calls are
required by common clock framework and this driver was missed while
converting all users of the Samsung original clocks driver to its
new implementation based on the common clock API.

Signed-off-by: Mateusz Krawczuk <m.krawczuk@partner.samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
[s.nawrocki@samsung.com: edited commit description]
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>