drm/i915: DSL_LINEMASK is 12 bits only on gen2

Gen3+ is 13 bits (12:0), and on gen2 only 12 (11:0). For both the high
bits are marked reserved, read-only so continue to mask them. Bit 31
is not reserved and has a meaning.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 6b41390..83b785f 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -849,15 +849,20 @@
 			     100))
 			DRM_DEBUG_KMS("pipe_off wait timed out\n");
 	} else {
-		u32 last_line;
+		u32 last_line, line_mask;
 		int reg = PIPEDSL(pipe);
 		unsigned long timeout = jiffies + msecs_to_jiffies(100);
 
+		if (IS_GEN2(dev))
+			line_mask = DSL_LINEMASK_GEN2;
+		else
+			line_mask = DSL_LINEMASK_GEN3;
+
 		/* Wait for the display line to settle */
 		do {
-			last_line = I915_READ(reg) & DSL_LINEMASK;
+			last_line = I915_READ(reg) & line_mask;
 			mdelay(5);
-		} while (((I915_READ(reg) & DSL_LINEMASK) != last_line) &&
+		} while (((I915_READ(reg) & line_mask) != last_line) &&
 			 time_after(timeout, jiffies));
 		if (time_after(jiffies, timeout))
 			DRM_DEBUG_KMS("pipe_off wait timed out\n");