drm/vmwgfx: Don't send drm sysfs hotplug events on initial master set

This may confuse user-space clients like plymouth that opens a drm
file descriptor as a result of a hotplug event and then generates a
new event...

Cc: <stable@vger.kernel.org>
Fixes: 5ea17348 ("drm/vmwgfx: Send a hotplug event at master_set")
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>