nv50,nvc0: expose R8/RG8_SRGB formats for texturing

This enables the GL_EXT_texture_sRGB_R8/RG8 extensions.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <!13769>
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