drm/client: Restrict the plane_state scope

The drm_client_modeset_commit_atomic function uses two times the
plane_state variable in inner blocks of code, but the variable has a scope
global to this function.

This will lead to inadvertent devs to reuse the variable in the second
block with the value left by the first, without any warning from the
compiler since value would have been initialized.

Fix this by moving the variable declaration to the proper scope.

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8bd9696ea915a4ad08be6d93a4d9565e8d6aa2f3.1560783090.git-series.maxime.ripard@bootlin.com