drm/i915: W/A for underruns with WM1+ disabled on icl

Disabling WM1+ on ICL causes tons of underruns with
linear/X-tiled framebuffers. We can avoid this by flipping
on a chicken bit affecting the way the hw fill the FIFO.
This may not be the final solution but should hopefully
avoid some underruns in the meantime.

v2: Apparently PIPE_CHICKEN is icl+ only
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190204202232.27153-1-ville.syrjala@linux.intel.comReviewed-by: Matt Roper <matthew.d.roper@intel.com>

drivers/gpu/drm/i915/i915_reg.h

@@ -7618,6 +7618,7 @@ enum {
 #define _PIPEB_CHICKEN			0x71038
 #define _PIPEC_CHICKEN			0x72038
 #define  PER_PIXEL_ALPHA_BYPASS_EN	(1 << 7)
+#define  PM_FILL_MAINTAIN_DBUF_FULLNESS	(1 << 0)
 #define PIPE_CHICKEN(pipe)		_MMIO_PIPE(pipe, _PIPEA_CHICKEN,\
 						   _PIPEB_CHICKEN)
 

drivers/gpu/drm/i915/intel_display.c

@@ -3911,6 +3911,12 @@ static void icl_set_pipe_chicken(struct intel_crtc *crtc)
 	 */
 	tmp |= PER_PIXEL_ALPHA_BYPASS_EN;
 
+	/*
+	 * W/A for underruns with linear/X-tiled with
+	 * WM1+ disabled.
+	 */
+	tmp |= PM_FILL_MAINTAIN_DBUF_FULLNESS;
+
 	I915_WRITE(PIPE_CHICKEN(pipe), tmp);
 }