meta: use _mesa_prepare_mipmap_levels()

meta: use _mesa_prepare_mipmap_levels()

The prepare_mipmap_level() wrapper for _mesa_prepare_mipmap_level() is
not needed.  It only served to undo the GL_TEXTURE_1D_ARRAY height/depth
change was was made before the call to prepare_mipmap_level()

Said another way, regardless of how the meta code manipulates the height/
depth dims for GL_TEXTURE_1D_ARRAY, the gl_texture_image dimensions are
correctly set up by _mesa_prepare_mipmap_levels().

Tested by plugging _mesa_meta_GenerateMipmap() into the swrast driver
and testing with piglit.

v2 (idr): Early out of the mipmap generation loop with dstImage is NULL.
This can occur for immutable textures that have a limited range of
levels or in the presense of memory allocation failures.  Fixes
arb_texture_view-mipgen on Intel platforms.

Reviewed-by: José Fonseca <jfonseca@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Tested-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>