drm/i915: Extract reading INSTDONE

INSTDONE is used in many places, and it varies from generation to
generation. This provides a good reason for us to extract the logic to
read the relevant information.

The patch has no functional change. It's prep for some new stuff.

v2: move the memset inside of i915_get_extra_instdone (Jani)
v3,4: bugs caught by (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>