staging: etnaviv: clean up prime import

Clean up the etnaviv prime import handling by combining msm_gem_import()
and msm_gem_prime_import_sg_table(), and then giving it an etnaviv_
prefix.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>