drm/msm: remove unused variable

After the drm_device_is_unplugged() was removed, the 'dev' variable is now
unused, and we get a warning for that:

drivers/gpu/drm/msm/msm_fbdev.c: In function 'msm_fbdev_mmap':
drivers/gpu/drm/msm/msm_fbdev.c:65:21: error: unused variable 'dev' [-Werror=unused-variable]

This removes the variable as well.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: e9f8250f ("drm/msm: remove the drm_device_is_unplugged check")
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1455181810-3910161-1-git-send-email-arnd@arndb.de