drm/i915: make intel_dp_compute_bpp static

Fix sparse warning:

drivers/gpu/drm/i915/intel_dp.c:1527:5: warning: symbol
'intel_dp_compute_bpp' was not declared. Should it be static?

Fixes: f9bb705e65f6 ("drm/i915: Update bits per component for display info")
Cc: Mika Kahola <mika.kahola@intel.com>
Cc: Jim Bride <jim.bride@linux.intel.com>
Reviewed-by: Jim Bride <jim.bride@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473946137-1931-1-git-send-email-jani.nikula@intel.com

drivers/gpu/drm/i915/intel_dp.c

@@ -1524,8 +1524,8 @@ void intel_dp_compute_rate(struct intel_dp *intel_dp, int port_clock,
 	}
 }
 
-int intel_dp_compute_bpp(struct intel_dp *intel_dp,
-			 struct intel_crtc_state *pipe_config)
+static int intel_dp_compute_bpp(struct intel_dp *intel_dp,
+				struct intel_crtc_state *pipe_config)
 {
 	int bpp, bpc;