nv50,nvc0: srgb rendering is only available for rgba/bgra

nv50,nvc0: srgb rendering is only available for rgba/bgra

Mark both L8_SRGB and L8A8_SRGB as non-renderable (the latter already
didn't have the bind flags). This makes the state tracker pick a
different format when rendering is required, or mark the fb as
incomplete. This fixes:

  bin/getteximage-formats init-by-clear-and-render -auto -fbo
  bin/getteximage-formats init-by-rendering -auto -fbo

which previously ran into srgb-encoding differences.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Cc: mesa-stable@lists.freedesktop.org