softpipe: Use mag texture filter also for clamped lod == 0

Since the LOD is clamped it is likely that we actually want magnification in this case, and if the LOD is truely 0 then we interpolate at the samples anyway making the linear filter acting like nearest neighbor filter.

While we're here also silence a potention varning about implicite float to double conversion.

Fixes a number of dEQP GLES2 and GLES3 test out of:
 dEQP-GLES2.functional.texture.filtering.*
 dEQP-GLES2.functional.texture.vertex.2d.filtering.*
 dEQP-GLES3.functional.texture.vertex.*.filtering.*
 dEQP-GLES3.functional.texture.filtering.*
 dEQP-GLES3.functional.texture.shadow.2d.*