drm/i915: Set the primary plane pipe select bits on gen4

i965 and g4x still have the pipe select bits in the plane control
registers, they're just hardcoded to select a specific pipe. However
plane C on i965 can still move between the pipes, thus we should
program the pipe select bits on i965 if we want to expose plane C
some day.

Since there is no harm in programming the bits on any plane on
i965/g4x let's just always set them. This will also make our
pre-computed register value match what the hardware register
would read, should we want to cross check the two.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180130203807.13721-2-ville.syrjala@linux.intel.com
Reviewed-by: Mika Kahola <mika.kahola@intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -3163,7 +3163,7 @@ static u32 i9xx_plane_ctl(const struct intel_crtc_state *crtc_state,
 	if (IS_HASWELL(dev_priv) || IS_BROADWELL(dev_priv))
 		dspcntr |= DISPPLANE_PIPE_CSC_ENABLE;
 
-	if (INTEL_GEN(dev_priv) < 4)
+	if (INTEL_GEN(dev_priv) < 5)
 		dspcntr |= DISPPLANE_SEL_PIPE(crtc->pipe);
 
 	switch (fb->format->format) {