st/mesa: Initialize tex_target in compile_tgsi_instruction

st/mesa: Initialize tex_target in compile_tgsi_instruction

Initialize to TGSI_TEXTURE_BUFFER (== 0), same as was done before the
variable type was changed to enum tgsi_texture_type.

Fixes a bunch of piglit failures with radeonsi, e.g.:

gles-3.0-transform-feedback-uniform-buffer-object: ../../../../src/gallium/auxiliary/tgsi/tgsi_util.c:502: tgsi_util_get_texture_coord_dim: Assertion `!"unknown texture target"' failed.

Corresponding compiler warning:

  CXX      state_tracker/st_glsl_to_tgsi.lo
../../../src/mesa/state_tracker/st_glsl_to_tgsi.cpp: In function ‘pipe_error st_translate_program(gl_context*, uint, ureg_program*, glsl_to_tgsi_visitor*, const gl_program*, GLuint, const ubyte*, const ubyte*, const ubyte*, const ubyte*, const ubyte*, GLuint, const ubyte*, const ubyte*, const ubyte*)’:
../../../src/mesa/state_tracker/st_glsl_to_tgsi.cpp:5992:23: warning: ‘tex_target’ may be used uninitialized in this function [-Wmaybe-uninitialized]
       ureg_memory_insn(ureg, inst->op, dst, num_dst, src, num_src,
       ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                        inst->buffer_access,
                        ~~~~~~~~~~~~~~~~~~~~
                        tex_target, inst->image_format);
                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../../src/mesa/state_tracker/st_glsl_to_tgsi.cpp:5866:27: note: ‘tex_target’ was declared here
    enum tgsi_texture_type tex_target;
                           ^~~~~~~~~~

Fixes: 9f9ce1625fb3 ("st/mesa: use TGSI enum types in st_glsl_to_tgsi.cpp")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>