drm/i915: fix i9xx irq enable/disable

Those functions are used on gen4 as well and gen4 does have a non-RCS
engine, so remove the BUG_ON and flip back the logic to what it was
before the ENGINE_READ/WRITE update

v2: update the posting read as well (Chris, Ville).

Fixes: baba6e57 ("drm/i915: take a reference to uncore in the engine and use it")
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190329165018.32953-1-daniele.ceraolospurio@intel.com