mesa: Validate target before resolving tex obj in glTex(ture)SubImageXD

mesa: Validate target before resolving tex obj in glTex(ture)SubImageXD

Currently, glTexSubImageXD attempt to resolve the texture object
(by calling _mesa_get_current_tex_object()) before validating the given
target. However, that method explicitly states that target must have been
validated before calling it, so it never returns a user error.

The target validation occurs later when texsubimage_error_check() is called.

This patch reorganizes target validation, taking it out from the error check
function and into a point before the texture object is resolved.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Cc: 10.6 <mesa-stable@lists.freedesktop.org>