drm/imagination: Remove firmware enable_reg

drm/imagination: Remove firmware enable_reg

After a previous commit ("drm/imagination: Mask GPU IRQs in threaded
handler"), this register is now only used to enable firmware interrupts at
start-of-day. This is, however, unnecessary since they are enabled by
default.

In addition, the soon-to-be-added RISC-V firmware processors do not have
an equivalent register.

Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Link: https://lore.kernel.org/r/20250410-sets-bxs-4-64-patch-v1-v6-8-eda620c5865f@imgtec.com
Signed-off-by: Matt Coster <matt.coster@imgtec.com>