fbdev: sh7760fb: Fix a possible memory leak in sh7760fb_alloc_mem()

When information such as info->screen_base is not ready, calling
sh7760fb_free_mem() does not release memory correctly. Call
dma_free_coherent() instead.

Fixes: 4a25e418 ("video: sh7760fb: SH7760/SH7763 LCDC framebuffer driver")
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Helge Deller <deller@gmx.de>