Clear pixmap->devPrivate.ptr [regression from 7c7294e]

ModifyPixmapHeader(pixdata = NULL) does not clear the
pixmap->devPrivate.ptr, instead the NULL value is interpreted as meaning
to keep the current value. (This is similar to the interpretation of the
other invalid values like depth=-1 which also mean not to change the
current property). However pixadata=NULL is indeed a valid value, and in
7c7294e, devPrivate.ptr == NULL was used to differentiate a bo pixmap
from a system pixmap. Except that we never did clear the ptr as
intended, and so X would immediately crash.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/intel_dri.c

@@ -91,6 +91,7 @@ static PixmapPtr get_front_buffer(DrawablePtr drawable, DrawablePtr *ret)
 						   0, 0,
 						   intel->front_pitch,
 						   NULL);
+			pixmap->devPrivate.ptr = NULL;
 			intel_set_pixmap_bo(pixmap, intel->front_buffer);
 		}
 	} else if (intel_get_pixmap_bo(pixmap)) {

src/intel_uxa.c

@@ -796,6 +796,7 @@ static Bool intel_uxa_put_image(PixmapPtr pixmap,
 						   w, h,
 						   0, 0,
 						   stride, NULL);
+			pixmap->devPrivate.ptr = NULL;
 			intel_set_pixmap_bo(pixmap, bo);
 			dri_bo_unreference(bo);
 
@@ -1039,6 +1040,7 @@ intel_uxa_create_pixmap(ScreenPtr screen, int w, int h, int depth,
 
 				list_del(&priv->in_flight);
 				screen->ModifyPixmapHeader(pixmap, w, h, 0, 0, stride, NULL);
+				pixmap->devPrivate.ptr = NULL;
 				intel_set_pixmap_private(pixmap, priv);
 				return pixmap;
 			}
@@ -1072,6 +1074,7 @@ intel_uxa_create_pixmap(ScreenPtr screen, int w, int h, int depth,
 		priv->tiling = tiling;
 
 		screen->ModifyPixmapHeader(pixmap, w, h, 0, 0, stride, NULL);
+		pixmap->devPrivate.ptr = NULL;
 
 		list_init(&priv->batch);
 		list_init(&priv->flush);
@@ -1108,6 +1111,7 @@ void intel_uxa_create_screen_resources(ScreenPtr screen)
 						   -1, -1,
 						   intel->front_pitch,
 						   NULL);
+			pixmap->devPrivate.ptr = NULL;
 		}
 		scrn->displayWidth = intel->front_pitch / intel->cpp;
 	}