drm/etnaviv: Add lockdep annotations for context lock

etnaviv_iommu_find_iova has it so etnaviv_iommu_insert_exact and
lockdep_assert_held should have it as well.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>