intel/compiler: Fix src0/desc setter ordering

intel/compiler: Fix src0/desc setter ordering

src0 vstride and type overlap with bits of the extended descriptor.
brw_set_desc() also sets the extended descriptor to 0.  So by setting
the descriptor, then setting src0, we were accidentally setting a bunch
of extended descriptor bits unintentionally.

When using this infrastructure for framebuffer writes (in a future
patch), this ended up setting the extended descriptor bit 20, which is
"Null Render Target" on Icelake, causing nothing to be written to the
framebuffer.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>