drm/vc4: v3d: Stop disabling interrupts

The vc4_irq_disable(), among other things, will call disable_irq() to
complete any in-flight interrupts.

This requires its counterpart, vc4_irq_enable(), to call enable_irq() which
causes issues addressed in a later patch.

However, vc4_irq_disable() is called by two callees: vc4_irq_uninstall()
and vc4_v3d_runtime_suspend().

vc4_irq_uninstall() also calls free_irq() which already disables the
interrupt line. We thus don't require an explicit disable_irq() for that
call site.

vc4_v3d_runtime_suspend() doesn't have any other code. However, the rest of
vc4_irq_disable() masks the interrupts coming from the v3d, so explictly
disabling the interrupt line is also redundant.

The only thing we really care about is thus to make sure we don't have any
handler in-flight, as suggested by the comment. We can thus replace
disable_irq() by synchronize_irq().

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>