drm: vmwgfx: remove drm_driver::master_set() return type

The function always returns zero (success). Ideally we'll remove it all
together - although that's requires a little more work.

For now, we can drop the return type and simplify the drm core code
surrounding it.

v2: remove redundant assignment (Sam)

Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Roland Scheidegger <sroland@vmware.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200530124640.4176323-1-emil.l.velikov@gmail.com