drivers/video: fsl-diu-fb: use an enum for the AOI index

Each of the five AOIs created by the DIU driver has a special purpose, and
they're not treated equally.  It makes sense to identify them with an enum
instead of a hard-coded number.

Since the 'index' is now an enum, it can only contain allowed values, so
there's no need to check for an invalid value.  This simplifies some other
code, such as fsl_diu_disable_panel(), which no longer needs to return an
error code.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>