mesa: Support flipping three-channel formats
Test system: POWER9 ppc64 (BE) system with a Radeon R5 230.
Before this commit, starting Xorg caused this message:
Assertion failed: !"Invalid array format" (../src/mesa/main/formats.c: _mesa_array_format_flip_channels: 421)
After this commit, Xorg starts successfully.
Note: I am not 100% sure this is correct; I took a best guess based on my understanding of Mesa formats. I don't see any obvious signs that this is wrong, running OpenGL software like Plasma or Trigger Rally.