staging: etnaviv: ensure cleanup of reservation object

The embedded reservation object is always initialised whenever we create
an etnaviv buffer object, but it is not always cleaned up.  Arrange this
to be clearer: always initialise the embedded reservation object
directly, and always clean the embedded reservation object up when
removing a buffer object.

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