freedreno: use drmPrime wrappers to import/export prime bo's

don't call drmIoctl() directly for prime bo's, use the wrappers instead.

v3: remove struct drm_prime_handle and split locking

Signed-off-by: Varad Gautam <varadgautam@gmail.com>
Signed-off-by: Rob Clark <robclark@freedesktop.org>