drm/lima: Clean up IRQ warnings

Use the optional form of platform_get_irq() for blocks that legitimately
may not be present, to avoid getting an annoying barrage of spurious
warnings for non-existent PPs on configurations like Mali-450 MP2.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/de475904091400ef6c123285f221094654d96d35.1587509150.git.robin.murphy@arm.com