drm: Remove struct drm_driver.gem_prime_mmap

All drivers initialize this field with drm_gem_prime_mmap(). Call
the function directly and remove the field. Simplifies the code and
resolves a long-standing TODO item.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230613150441.17720-3-tzimmermann@suse.de