drm/vmwgfx: Fix error paths when mapping framebuffer

Rather than returning immediately, make sure to unlock the
mutexes first.

Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reported-by: Emil Velikov <emil.l.velikov@gmail.com>
Cc: <stable@vger.kernel.org>