drm/i915: extract intel_gpu_reset

Slightly cleans up the code and could be useful for e.g. Ben
Widawsky's hw context patches.

v2: New colours!

Cc: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>