tgsi: use truncf in micro_trunc

tgsi: use truncf in micro_trunc

Switches to using truncf in micro_trunc.

Fixes the following piglit tests (for softpipe):

/spec/glsl-1.30/execution/built-in-functions/...
fs-trunc-float
fs-trunc-vec2
fs-trunc-vec3
fs-trunc-vec4
vs-trunc-float
vs-trunc-vec2
vs-trunc-vec3
vs-trunc-vec4

/spec/glsl-1.50/execution/built-in-functions/...
gs-trunc-float
gs-trunc-vec2
gs-trunc-vec3
gs-trunc-vec4

Signed-off-by: Lars Hamre <chemecse@gmail.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>