drm/i915: gen2: move error capture of IER to its correct place

While checking the error capture path I noticed that this register is
read twice for GEN2, so fix this and also move the read where it's done
for other platforms.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>