drm/etnaviv: replace dma_resv_get_excl_unlocked

We certainly hold the reservation lock here, no need for the RCU dance.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211025080532.177424-3-christian.koenig@amd.com