staging: fbtft: fb_ssd1306: Refactor write_vmem()

Refactor write_vmem() for sake of readability.

While here, fix indentation in one comment.

Acked-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>