radeonsi: force level zero on image instructions in non-fragment shaders (v2)

radeonsi: force level zero on image instructions in non-fragment shaders (v2)

Section 8.9 (Texture Functions) of the OpenGL Shading Language 4.5
specification:

   However, automatic level of detail is computed only for fragment shaders.
   Other shaders operate as though the base level of detail were computed as
   zero.

and Section 8.9.3 (Texture Gather Functions):

   When performing a texture gather operation, the minification and
   magnification filters are ignored, and the rules for LINEAR filtering in
   the OpenGL Specification are applied to the base level of the texture
   image to identify the four texels i_0 j_1, i_1 j_1, i_1 j_0, and i_0 j_0.

Of course, explicit LOD or derivative variants work in all shader types.

This fixes several GL4x-CTS.texture_gather.* tests.

v2: TG4 is always level zero (thanks, Ilia)
Reviewed-by: Marek Olšák <marek.olsak@amd.com>