[media] s5p-g2d: Fix error handling

According to the surrounding goto, it is likely that 'unprep_clk_gate'
was expected here.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>