drm/prime: Unexport helpers for fd/handle conversion

Unexport drm_gem_prime_fd_to_handle() and drm_gem_prime_handle_to_fd().
Both are only used internally within the PRIME code.

v2:
	* reword docs as functions are now unexported (Simon)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Simon Ser <contact@emersion.fr>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230620080252.16368-4-tzimmermann@suse.de