v3dv: don't swap R/B channels for VK_FORMAT_R5B6G5_UNORM_PACK16

v3dv: don't swap R/B channels for VK_FORMAT_R5B6G5_UNORM_PACK16

This corresponds to PIPE_FORMAT_B5G6R5_UNORM, which is the format that
is natively supported. Also, we can't swap R/B on 3-channel images!

Also, we should rely on the v3dv format table for this rather than
pipe format descriptions since we specify the expected correct swizzles
there for all supported formats. This, for example, gets us correct
beahvior for things like VK_FORMAT_B4G4R4A4_UNORM_PACK16 without
needing to special case it.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>