egl_dri2: swrastGetDrawableInfo: set *x, *y [v2]

egl_dri2: swrastGetDrawableInfo: set *x, *y [v2]

In swrastGetDrawableInfo, set *x and *y, not just *w and *h;
this fixes a crash later in drisw_update_tex_buffer when the
(formerly) uninitialized x and y values are used to construct
an address in a call to llvmpipe_transfer_map.

Fixes crash in Piglit test
"spec@egl 1.4@eglcreatepbuffersurface and then glclear"
(<piglit dir>/bin/egl-create-pbuffer-surface -auto)
that occurred intermittently, e.g. when the uninitialized x and y in
drisw_update_tex_buffer just happened to contain absurd non-zero values.

v2: Initialize in case if function succeeds or fails, just like *w/*h.

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Ben Crocker <bcrocker@redhat.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>