r300: properly initialize new_vs in r300_draw_init_vertex_shader

r300: properly initialize new_vs in r300_draw_init_vertex_shader

Fixes the following valgrind warnings:
 Conditional jump or move depends on uninitialised value(s)
    at 0x5D1298C: draw_pt_so_emit_prepare (draw_pt_so_emit.c:95)
    by 0x5D89FD7: llvm_middle_end_prepare (draw_pt_fetch_shade_pipeline_llvm.c:319)
    by 0x5D13BFF: vsplit_prepare (draw_pt_vsplit.c:229)
    by 0x5D0D5D8: draw_pt_arrays.isra.0 (draw_pt.c:124)
    by 0x5D0DA08: draw_instances (draw_pt.c:484)
    by 0x5D0DEB9: draw_vbo (draw_pt.c:610)
    by 0x5E847D6: r300_swtcl_draw_vbo (r300_render.c:901)
    by 0x5D67125: u_vbuf_draw_vbo (u_vbuf.c:1470)
    by 0x5CFCAD3: cso_multi_draw (cso_context.c:1639)
    by 0x58FEC37: st_draw_gallium (st_draw.c:186)
    by 0x5A486BA: _mesa_draw_arrays.part.0 (draw.c:1323)
    by 0x48B4E27: stub_glDrawArrays (piglit-dispatch-gen.c:12421)
  Uninitialised value was created by a stack allocation
    at 0x5E90D3F: r300_draw_init_vertex_shader (r300_vs_draw.c:313)

 Conditional jump or move depends on uninitialised value(s)
    at 0x5D175D9: draw_create_vertex_shader (draw_vs.c:70)
    by 0x5E90E7A: r300_draw_init_vertex_shader (r300_vs_draw.c:366)
    by 0x5E8751F: r300_create_vs_state (r300_state.c:1950)
    by 0x594453B: st_create_common_variant (st_program.c:888)
    by 0x594720C: st_get_common_variant (st_program.c:973)
    by 0x59484B6: st_precompile_shader_variant (st_program.c:1994)
    by 0x59484B6: st_finalize_program (st_program.c:2056)
    by 0x58F0571: st_program_string_notify (st_cb_program.c:128)
    by 0x5928C9B: st_link_tgsi (st_glsl_to_tgsi.cpp:7514)
    by 0x5904B53: st_link_shader (st_glsl_to_ir.cpp:178)
    by 0x58D3E08: _mesa_glsl_link_shader (link_program.cpp:91)
    by 0x58865C7: link_program (shaderapi.c:1363)
    by 0x58865C7: link_program_error.part.0 (shaderapi.c:1474)
    by 0x48DBC5A: stub_glLinkProgram (piglit-dispatch-gen.c:34426)
  Uninitialised value was created by a stack allocation
    at 0x5E90D4F: r300_draw_init_vertex_shader (r300_vs_draw.c:313)

Based on Filip Gawin's
   r300: set new_vs.type to PIPE_SHADER_IR_TGSI

CC: mesa-stable
Reviewed-by: Emma Anholt <emma@anholt.net>
Signed-off-by: Pavel Ondračka <pavel.ondracka@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14620>