drivers/video: fsl-diu-fb: improve local variable usage in some functions

Clean up the local variable usage in request_irq_local() and allocate_buf().
This streamlines the code without affecting functionality.

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