drm/v3d: Use platform_get_irq_optional() to get optional IRQs

Aside from being more correct, the non optional version of the function
prints an error when failing to find the IRQ.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/1608755714-18233-2-git-send-email-stefan.wahren@i2se.com