drm/etnaviv: Improve unlocking of a mutex in etnaviv_iommu_map_gem()

Add a jump target so that a call of the function "mutex_unlock" is stored
only once at the end of this function implementation.
Replace three calls by goto statements.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>