drm/i915: Fix a couple of "should it be static?" sparse warnings

A genuine 'static' omission and 2 other warnings triggered by not
including the header where those functions where defined.
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -30,6 +30,7 @@
 #include "intel_drv.h"
 #include "../../../platform/x86/intel_ips.h"
 #include <linux/module.h>
+#include <drm/i915_powerwell.h>
 
 #define FORCEWAKE_ACK_TIMEOUT_MS 2
 
@@ -2465,7 +2466,7 @@ static void hsw_compute_wm_results(struct drm_device *dev,
 
 /* Find the result with the highest level enabled. Check for enable_fbc_wm in
  * case both are at the same level. Prefer r1 in case they're the same. */
-struct hsw_wm_values *hsw_find_best_result(struct hsw_wm_values *r1,
+static struct hsw_wm_values *hsw_find_best_result(struct hsw_wm_values *r1,
 						  struct hsw_wm_values *r2)
 {
 	int i, val_r1 = 0, val_r2 = 0;