i965: Don't use GCC extension for ?: with only two operands.

From the "apparently I don't know C" files...GCC apparently supports:

    x ?: y

which is equivalent to

    x ? x : y

except that it doesn't cause side-effects to occur twice.  See:
https://gcc.gnu.org/onlinedocs/gcc/Conditionals.html#Conditionals

This was confusing and looked like a typo.  It doesn't really buy us
anything, so just write the obvious code in normal C.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>

src/mesa/drivers/dri/i965/intel_fbo.c

@@ -551,10 +551,12 @@ intel_renderbuffer_update_wrapper(struct brw_context *brw,
 
    irb->mt_layer = layer_multiplier * layer;
 
-   if (layered) {
-      irb->layer_count = image->TexObject->NumLayers ?: mt->level[level].depth / layer_multiplier;
-   } else {
+   if (!layered) {
       irb->layer_count = 1;
+   } else if (image->TexObject->NumLayers > 0) {
+      irb->layer_count = image->TexObject->NumLayers;
+   } else {
+      irb->layer_count = mt->level[level].depth / layer_multiplier;
    }
 
    intel_miptree_reference(&irb->mt, mt);