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>
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index a7bf31a..f678017 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -3277,17 +3277,12 @@
 	     "bogus fence setup with stride: 0x%x, tiling mode: %i\n",
 	     obj->stride, obj->tiling_mode);
 
-	switch (INTEL_INFO(dev)->gen) {
-	case 9:
-	case 8:
-	case 7:
-	case 6:
-	case 5:
-	case 4: i965_write_fence_reg(dev, reg, obj); break;
-	case 3: i915_write_fence_reg(dev, reg, obj); break;
-	case 2: i830_write_fence_reg(dev, reg, obj); break;
-	default: BUG();
-	}
+	if (IS_GEN2(dev))
+		i830_write_fence_reg(dev, reg, obj);
+	else if (IS_GEN3(dev))
+		i915_write_fence_reg(dev, reg, obj);
+	else if (INTEL_INFO(dev)->gen >= 4)
+		i965_write_fence_reg(dev, reg, obj);
 
 	/* And similarly be paranoid that no direct access to this region
 	 * is reordered to before the fence is installed.