gallium/ttn: don't upset nir_validate w/ BRK's

gallium/ttn: don't upset nir_validate w/ BRK's

Previously we were unconditionally doing ttn_get_src() even for
instructions with no src's.  Which created a lot of unnecessary
load_const instructions.  These were mostly harmless since NIR opt
passes would strip them back out.  But for an ENDIF following a
BRK, it would result in load_const instructions created after the
NIR break instruction.  Which nir_validate dislikes.

But we can actually just dtrt by using NumSrcRegs instead.

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Eric Anholt <eric@anholt.net>