i965/fs: Initialize output_components[] by filling it with zeros.

Prior to commit afaf5b59e, emit_fb_writes() looped from 0 to 3, writing
all four components of a vec4 color output.  However, that broke for
smaller output types (float, vec2, or vec3).  To fix that, I introduced
a new variable (output_components[]) containing the size of the output
type for each render target.

Unfortunately, I forgot to actually initialize it in the constructor,
which meant that unless a shader wrote to gl_FragColor, or the specific
output for each render target, output_components would contain a garbage
value, and we'd loop for a completely non-deterministic amount of time.

Not actually emitting any color writes seems like the right approach.
We may still need to emit a render target write (to terminate the
thread), but don't have to put in any sensible values (the shader didn't
write anything, after all).

Fixes a regression since afaf5b59e4.

Backported from master commit 6928bea7ca.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=54193
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
Tested-by: Ian Romanick <idr@freedesktop.org>

src/mesa/drivers/dri/i965/brw_fs.h

@@ -410,6 +410,7 @@ public:
 
       this->frag_depth = NULL;
       memset(this->outputs, 0, sizeof(this->outputs));
+      memset(this->output_components, 0, sizeof(this->output_components));
       this->first_non_payload_grf = 0;
       this->max_grf = intel->gen >= 7 ? GEN7_MRF_HACK_START : BRW_MAX_GRF;