drm/i915: move module parameters into a struct, in a new file

With 20+ module parameters, I think referring to them via a struct
improves clarity over just having a bunch of globals. While at it, move
the parameter initialization and definitions into a new file
i915_params.c to reduce clutter in i915_drv.c.

Apart from the ill-named i915_enable_rc6, i915_enable_fbc and
i915_enable_ppgtt parameters, for which we lose the "i915_" prefix
internally, the module parameters now look the same both on the kernel
command line and in code. For example, "i915.modeset".

The downsides of the change are losing static on a couple of variables
and not having the initialization and module_param_named() right next to
each other. On the other hand, all module parameters are now defined in
one place at i915_params.c. Plus you can do this to find all module
parameter references:

$ git grep "i915\." -- drivers/gpu/drm/i915

v2:
- move the definitions into a new file
- s/i915_params/i915/
- make i915_try_reset i915.reset, for consistency

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_lvds.c

@@ -848,8 +848,8 @@ static bool compute_is_dual_link_lvds(struct intel_lvds_encoder *lvds_encoder)
 	struct drm_i915_private *dev_priv = dev->dev_private;
 
 	/* use the module option value if specified */
-	if (i915_lvds_channel_mode > 0)
-		return i915_lvds_channel_mode == 2;
+	if (i915.lvds_channel_mode > 0)
+		return i915.lvds_channel_mode == 2;
 
 	if (dmi_check_system(intel_dual_link_lvds))
 		return true;
@@ -1036,7 +1036,7 @@ void intel_lvds_init(struct drm_device *dev)
 					intel_find_panel_downclock(dev,
 					fixed_mode, connector);
 				if (intel_connector->panel.downclock_mode !=
-					NULL &&	i915_lvds_downclock) {
+					NULL &&	i915.lvds_downclock) {
 					/* We found the downclock for LVDS. */
 					dev_priv->lvds_downclock_avail = true;
 					dev_priv->lvds_downclock =