tgsi: fix incorrect tgsi_shader_info::num_tokens computation

tgsi: fix incorrect tgsi_shader_info::num_tokens computation

We were incrementing num_tokens in each loop iteration while parsing
the shader.  But each call to tgsi_parse_token() can consume more than
one token (and often does).  Instead, just call the tgsi_num_tokens()
function.

Luckily, this issue doesn't seem to effect any current users of this
field (llvmpipe just checks for <= 1, for example).

Reviewed-by: Neha Bhende<bhenden@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>