common: Correct texture init for meta pbo uploads and downloads.

common: Correct texture init for meta pbo uploads and downloads.

This moves the line setting immutability for the texture to after
_mesa_initialize_texture_object so that the initializer function will not
cancel it out. Moreover, because of the ARB_texture_view extension, immutable
textures must have NumLayers > 0, or depth will equal (0-1)=0xFFFFFFFF during
SURFACE_STATE setup, which triggers assertions.

v2: Review from Kenneth Graunke:
   - Include more explanation in the commit message.
   - Make texture setup bug fixes into a separate patch.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Cc: "10.4, 10.5" <mesa-stable@lists.freedesktop.org>