drm/etnaviv: Implement mmap as GEM object function

Moving the driver-specific mmap code into a GEM object function allows
for using DRM helpers for various mmap callbacks.

The respective etnaviv functions are being removed. The file_operations
structure fops is now being created by the helper macro
DEFINE_DRM_GEM_FOPS().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>