mesa: handle some srgb cases in the fast path in _mesa_get_teximage

mesa: handle some srgb cases in the fast path in _mesa_get_teximage

Previously, always did unorm8->float/nonlinear-to-linear conversion (using
lookup table), then convert back to nonlinear (using the expensive math
func pow among others), and finally convert back to int (assuming caller
wants unorm8), because the float texture fetch function is used for getting
the actual texel values. This should probably all be changed at some point,
but for now simply enable the memcpy path also for srgb formats (but if for
instance swizzling is required, still the whole conversion will be done).