drm/i915: do not read swizzle info if unavailable

Since gen8 we don't use swizzle anymore. Don't dump registers related to
it: registers may or may not be there.

v2: pull the rest of driver state reporting before the read out (Chris)

Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@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/20200702200714.1278-1-lucas.demarchi@intel.com