drm: error out with EBUSY when device has existing master

As requested by Adam, provide different error message for when the
device has an existing master. An audit of the following projects, shows
that the errno is used only for printf() purposes.

xorg/xserver
xorg/drivers/xf86-video-ati
xorg/drivers/xf86-video-amdgpu
xorg/drivers/xf86-video-intel
xorg/drivers/xf86-video-tegra
xorg/drivers/xf86-video-freedreno
xorg/drivers/xf86-video-nouveau
xorg/drivers/xf86-video-vmwgfx

qt/kwin/plasma
gtk/mutter/gnomeshell
efl/enlightment

Cc: Adam Jackson <ajax@redhat.com>
Suggested-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200319172930.230583-2-emil.l.velikov@gmail.com