rk_fb: sysfs: add dump_buffer func to fb sysfs

rk_fb: sysfs: add dump_buffer func to fb sysfs

Due to some time we want to know which buffer vop scaning, use "io"
cammand to dump buffer is too complex, so we add a sys node to help
buffer dump.

how to use it:
- echo bin > /sys/class/graphics/fb0/disp_info
  it will create bin file at /data/xxx.bin
- or echo bmp > /sys/class/graphics/fb0/disp_info
  it will create bmp file at /data/xxx.bmp,
  this file is normal bmp file.

Signed-off-by: Mark Yao <mark.yao@rock-chips.com>