softpipe: set a lower minimum clamp value for texture coordinate border clamp

softpipe: set a lower minimum clamp value for texture coordinate border clamp

The value of -0.5f is not small enough to produce negative coordinates,
so lower the minimum clamp value to -1.0f. This fixes a number of tests
from
   dEQP-GLES31.functional.texture.border_clamp.*

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>