mesa: Unconditionally enable floating-point textures

mesa: Unconditionally enable floating-point textures

ARB_texture_float references US Patent #6,650,327 [1] which has a filing date
of June 16 1998.

According to [2], patents filed after 1995 expire 20 years from the filing
date, giving an expiration of June 17 2018.

[1] https://www.google.com/patents/US6650327
[2] https://en.wikipedia.org/wiki/Term_of_patent_in_the_United_States

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>