nvc0: fix blitting red to srgb8_alpha

For some reason the 2d engine can't handle this. Red formats get special
treatment there, so perhaps related.

Fixes dEQP-GLES3 tests of the form:

  dEQP-GLES3.functional.fbo.blit.conversion.r{8,16f,32f}_to_srgb8_alpha8

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit 92f56fbd)