drm/i915: Factor out intel_tile_width()

Pull the tile width calculations from intel_fb_stride_alignment() into a
new function intel_tile_width().

Also take the opportunity to pass aroun dev_priv instead of dev to
intel_fb_stride_alignment().

v2: Reorder argumnents to be more consistent with other functions
    Change intel_fb_stride_alignment() to accept dev_priv instead of dev
v3: Deal with Y tilling (Daniel)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1452625717-9713-3-git-send-email-ville.syrjala@linux.intel.com
diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 0155382..9a8075b 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -1070,8 +1070,8 @@
 				   uint64_t fb_format_modifier);
 void intel_fb_obj_flush(struct drm_i915_gem_object *obj, bool retire,
 			enum fb_op_origin origin);
-u32 intel_fb_stride_alignment(struct drm_device *dev, uint64_t fb_modifier,
-			      uint32_t pixel_format);
+u32 intel_fb_stride_alignment(const struct drm_i915_private *dev_priv,
+			      uint64_t fb_modifier, uint32_t pixel_format);
 
 /* intel_audio.c */
 void intel_init_audio(struct drm_device *dev);