drm/vc4: hvs: Test if the EOF interrupts are enabled

We currently enable the EOF interrupts through the CRTC destroy_state
implementation.

However, nothing guarantees that we can't call destroy_state multiple
times in a row, and therefore before the EOF interrupt even happens.

This means we would enable the interrupt multiple times but disable it
only once. It wasn't an issue so far since the interrupts were only
enabled by setting a bit in a register, but with BCM2712 we will use an
external interrupt controller, with a refcounted interrupt.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>