drm/imagination: Disable interrupts before suspending the GPU

drm/imagination: Disable interrupts before suspending the GPU

This is an additional safety layer to ensure no accesses to the GPU
registers can be made while it is powered off.

While we can disable IRQ generation from GPU, META firmware, MIPS
firmware and for safety events, we cannot do the same for the RISC-V
firmware.
To keep a unified approach, once the firmware has completed its power
off sequence, disable IRQs for the while GPU at the kernel level
instead.

Signed-off-by: Alessio Belle <alessio.belle@imgtec.com>
Reviewed-by: Matt Coster <matt.coster@imgtec.com>
Link: https://patch.msgid.link/20260310-drain-irqs-before-suspend-v1-2-bf4f9ed68e75@imgtec.com
Signed-off-by: Matt Coster <matt.coster@imgtec.com>