vga_switcheroo: Use pr_fmt()

Use pr_fmt() to define the "vga_switcheroo: " prefix that is prepended
to all output messages emitted by pr_*() functions. This allows making
existing strings much shorter and eliminates a bunch of warnings from
checkpatch about lines being overly long.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>