video: da8xx-fb rev2: fix disabling of palette completion interrupt

Writing '1' to particular bit of IRQENABLE_CLEAR register disables the
corresponding interrupt on revision 2 LCDC. This register was wrongly
configured to disable all previous enabled interrupts instead of
disabling only palette completion interrupt. Patch fixes it by clearing
only palette completion interrupt bit.

Signed-off-by: Manjunathappa, Prakash <prakash.pm@ti.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>