drm/i915: clear error registers after error capture

We need to clear the register in order to get correct value after the
next potential hang.

v2: Centralize error register clearing in i915_irq.c (Chris)

v3: Don't read gen8 register on < gen6 (Chris)

v4: Don't swap gen8+ & gen6+ code... (Chris)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180830132424.21940-1-lionel.g.landwerlin@intel.com