swrast: Fix rgba_draw_pixels with GL_COLOR_INDEX

swrast: Fix rgba_draw_pixels with GL_COLOR_INDEX

When we implemented the format conversion rewrite we forgot to handle
GL_COLOR_INDEX here, which needs special handling.

Fixes the following piglit test:
bin/gl-1.0-drawpixels-color-index -auto -fbo

Buzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90213

Tested-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>