meta/blit: Write depth only when asked for

meta/blit: Write depth only when asked for

Implementing an idea from Ken, on i965 the shader program for 2D
blits becomes significantly simpler.

Before:

pln(8)   g6<1>F    g4<0,1,0>F    g2<8,8,1>F  { align1 1Q compacted };
pln(8)   g7<1>F    g4.4<0,1,0>F  g2<8,8,1>F  { align1 1Q compacted };
send(8)  g2<1>UW   g6<8,8,1>F
         sampler (1, 0, 0, 1) mlen 2 rlen 4  { align1 1Q };
mov(8)   g123<1>F  g2<8,8,1>F                { align1 1Q compacted };
mov(8)   g124<1>F  g3<8,8,1>F                { align1 1Q compacted };
mov(8)   g125<1>F  g4<8,8,1>F                { align1 1Q compacted };
mov(8)   g126<1>F  g5<8,8,1>F                { align1 1Q compacted };
mov(8)   g127<1>F  g2<8,8,1>F                { align1 1Q compacted };
nop                                                             ;
sendc(8) null        g123<8,8,1>F
    render RT write SIMD8 LastRT Surface = 0 mlen 5 rlen 0 { align1 1Q EOT };

After:

pln(8)   g6<1>F     g4<0,1,0>F    g2<8,8,1>F   { align1 1Q compacted };
pln(8)   g7<1>F     g4.4<0,1,0>F  g2<8,8,1>F   { align1 1Q compacted };
send(8)  g124<1>UW  g6<8,8,1>F
         sampler (1, 0, 0, 1) mlen 2 rlen 4    { align1 1Q };
sendc(8) null        g124<8,8,1>F
   render RT write SIMD8 LastRT Surface = 0 mlen 4 rlen 0 { align1 1Q EOT };

v2 (Matt): Removed unintended white-space change

Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>