mesa: Fix extension dependency for half-float TexBOs

mesa: Fix extension dependency for half-float TexBOs

Half-float TexBOs should require both GL_ARB_half_float_pixel and
GL_ARB_texture_float.  This doesn't matter much in practice.  Every
driver that supports GL_ARB_texture_buffer_object already supports
GL_ARB_half_float_pixel.  We only expose the TexBO extension in core
profiles, and those require GL_ARB_texture_float.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>