fbdev: i.MX31: fix panning, error handling, clean up

1. check for errors returned from clk_get()
2. fix "Unbalanced enable for IRQ 160"
3. fix transmit descriptor handling in panning
4. clean frame buffer on blank - useful for OLED displays
5. formatting clean up

Signed-off-by: Guennadi Liakhovetski <lg@denx.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>