radv/llvm: fix enabled_channels for compressed exports

radv/llvm: fix enabled_channels for compressed exports

The old values seemed to work fine, but the ISA docs recommend 0x0,0x3,0xc
and 0xf:

COMPR==1: export half-dword enable. Valid values are: 0x0,3,c,f
[0] enables VSRC0 : R,G from one VGPR (R in low bits, G high)
[2] enables VSRC1 : B,A from one VGPR (B in low bits, A high)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9459>