drm/i915: fix crash in error state readout on non-execlist platforms v2

Looks like this was introduced in:
commit d1675198ed1f21aec6e036336e4340c40b726497
Author: Alex Dai <yu.dai@intel.com>
Date:   Wed Aug 12 15:43:43 2015 +0100

    drm/i915: Integrate GuC-based command submission

This patch assumed LRC contexts and HWS layout, which is incorrect on
platforms without execlists.  This can lead to a crash in GPU error
state readout on those platforms.

I don't see a bug filed for this, but there may be one that I haven't
found.

v2: fixup offset handling for error capture fix (Dave)

Cc: Alex Dai <yu.dai@intel.com>
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gpu_error.c

@@ -457,17 +457,23 @@ int i915_error_state_to_str(struct drm_i915_error_state_buf *m,
 		}
 
 		if ((obj = error->ring[i].hws_page)) {
+			u64 hws_offset = obj->gtt_offset;
+			u32 *hws_page = &obj->pages[0][0];
+
+			if (i915.enable_execlists) {
+				hws_offset += LRC_PPHWSP_PN * PAGE_SIZE;
+				hws_page = &obj->pages[LRC_PPHWSP_PN][0];
+			}
 			err_printf(m, "%s --- HW Status = 0x%08llx\n",
-				dev_priv->ring[i].name,
-				obj->gtt_offset + LRC_PPHWSP_PN * PAGE_SIZE);
+				   dev_priv->ring[i].name, hws_offset);
 			offset = 0;
 			for (elt = 0; elt < PAGE_SIZE/16; elt += 4) {
 				err_printf(m, "[%04x] %08x %08x %08x %08x\n",
 					   offset,
-					   obj->pages[LRC_PPHWSP_PN][elt],
-					   obj->pages[LRC_PPHWSP_PN][elt+1],
-					   obj->pages[LRC_PPHWSP_PN][elt+2],
-					   obj->pages[LRC_PPHWSP_PN][elt+3]);
+					   hws_page[elt],
+					   hws_page[elt+1],
+					   hws_page[elt+2],
+					   hws_page[elt+3]);
 					offset += 16;
 			}
 		}