drivers: panel: latona: code clean up and fixes

* Remove unneeded code and fix compiler warning
* Beautify code.
* Remove aat1402_set_brightness() from poweron sequences.
  This makes the use of a global var to store the current LCD
  brightness unnecessary. The brigthness will be updated anyway.
* The panel is initialized by the bootloader, there's no need to
  make the code dependant on CONFIG_FB_OMAP_BOOTLOADER_INIT.
* Change default brightness to 108.
* Use dev_dbg when possible.