drm/doc: Clarify PRIME documentation

PRIME fds aren't actually GEM fds but are (like the modeset API)
independent of the underlying buffer manager, as long as that one uses
uint32_t as handles. So move that entire section out of the GEM
section and reword it a bit to clarify which parts of PRIME are
generic, and which are the mandatory pieces for GEM drivers to
correctly implement the GEM lifetime rules. The rewording mostly
consists of not mixing up GEM, PRIME and DRM.

I've considered adding some blurbs to the GEM object lifetime section
about interactions with dma-bufs, but then dropped that. As long as
drivers use the right helpers they should have this all implemented
correctly and hence can be regarded as an implementation detail of the
PRIME/GEM helpers. So no need to confuse driver writers with those
tricky interactions.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>