drm/i915: don't write powered down IRQ registers on Gen 8

drm/i915: don't write powered down IRQ registers on Gen 8

If we enable unclaimed register reporting on Gen 8, we will discover
that the IRQ registers for pipes B and C are also on the power well,
so writes to them when the power well is disabled result in unclaimed
register errors.

Also, hsw_power_well_post_enable() already takes care of re-enabling
them once the power well is enabled.

Testcase: igt/pm_rpm/rte
Signed-off-by: Paulo Zanoni <[email protected]>
Reviewed-by: Rodrigo Vivi <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>