drm/i915: Organize Fence registers for future enablement.

drm/i915: Organize Fence registers for future enablement.

Let's be optimistic that for future platforms this will remain the same
and reorg a bit.
This reorg in if blocks instead of switch make life easier for future
platform support addition.

v2: Jani pointed out I was missing reg_830 for some gen3 platforms. So let's make
    this platforms subcases of Gen checks.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>