nvc0: always emit a full shader colormask

nvc0: always emit a full shader colormask

Indications are that if the colormask indicates a single bit set on
fermi, that value will always be read from $r0 instead of a potentially
higher register (if e.g. green is set). Not to upset the counting logic,
always set the header up with a full color mask for each RT. Such a
situation can basically only ever happen with generated blit shaders.

Fixes the following piglit on Fermi (Kepler is unaffected):
  fbo-stencil blit GL_DEPTH32F_STENCIL8

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "10.6 11.0" <mesa-stable@lists.freedesktop.org>