gallium/swr: Fix depth values for blit scenario
This fixes the following piglit tests:
- spec/!opengl 1.1/copyteximage 1d
- spec/!opengl 1.1/copyteximage 2d
- spec/!opengl 1.1/depthstencil-default_fb-copypixels
- spec/!opengl 1.4/copy-pixels
- spec/arb_depth_buffer_float/fbo-depth-gl_depth32f_stencil8-copypixels
- spec/arb_depth_buffer_float/fbo-depthstencil-gl_depth32f_stencil8-copypixels
- spec/arb_depth_buffer_float/fbo-generatemipmap-formats/gl_depth_component32f
- spec/arb_depth_buffer_float/fbo-generatemipmap-formats/gl_depth_component32f npot
- spec/arb_depth_texture/fbo-generatemipmap-formats/gl_depth_component
- spec/arb_depth_texture/fbo-generatemipmap-formats/gl_depth_component npot
- spec/arb_depth_texture/fbo-generatemipmap-formats/gl_depth_component16
- spec/arb_depth_texture/fbo-generatemipmap-formats/gl_depth_component16 npot
- spec/arb_depth_texture/fbo-generatemipmap-formats/gl_depth_component24
- spec/arb_depth_texture/fbo-generatemipmap-formats/gl_depth_component24 npot
- spec/arb_depth_texture/fbo-generatemipmap-formats/gl_depth_component32
- spec/arb_depth_texture/fbo-generatemipmap-formats/gl_depth_component32 npot
- spec/arb_depth_texture/texdepth
- spec/arb_framebuffer_object/arb_framebuffer_object-depth-stencil-blit depth gl_depth24_stencil8
- spec/arb_framebuffer_object/arb_framebuffer_object-depth-stencil-blit depth gl_depth32f_stencil8
- spec/arb_texture_cube_map/copyteximage cube
- spec/arb_texture_rectangle/copyteximage rect
- spec/ext_framebuffer_multisample/accuracy all_samples depth_resolve depthstencil
- spec/ext_framebuffer_multisample/accuracy all_samples depth_resolve small depthstencil
- spec/ext_packed_depth_stencil/fbo-blit-d24s8
- spec/ext_packed_depth_stencil/fbo-depth-gl_depth24_stencil8-copypixels
- spec/ext_packed_depth_stencil/fbo-depthstencil-gl_depth24_stencil8-copypixels
- spec/ext_texture_array/copyteximage 1d_array
- spec/ext_texture_array/copyteximage 2d_array