video: fbdev: au1100fb: Drop unnecessary NULL ptr check

clk_disable() already checks the clk ptr using IS_ERR_OR_NULL(clk) and
clk_enable() checks the clk ptr using !clk, so there is no need to check clk
ptr again before calling them.

Signed-off-by: Yihao Han <hanyihao@vivo.com>
Signed-off-by: Helge Deller <deller@gmx.de>