vgaarb: cleanup vgaarb.h

Merge the different CONFIG_VGA_ARB ifdef blocks, remove superflous
externs, and regularize the stubs for !CONFIG_VGA_ARB.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210716061634.2446357-5-hch@lst.de


Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>