drm/i915/gem: Add an intermediate proto_context struct (v5)

The current context uAPI allows for two methods of setting context
parameters: SET_CONTEXT_PARAM and CONTEXT_CREATE_EXT_SETPARAM.  The
former is allowed to be called at any time while the later happens as
part of GEM_CONTEXT_CREATE.  Currently, everything settable via one is
settable via the other.  While some params are fairly simple and setting
them on a live context is harmless such the context priority, others are
far trickier such as the VM or the set of engines.  In order to swap out
the VM, for instance, we have to delay until all current in-flight work
is complete, swap in the new VM, and then continue.  This leads to a
plethora of potential race conditions we'd really rather avoid.

Unfortunately, both methods of setting the VM and the engine set are in
active use today so we can't simply disallow setting the VM or engine
set vial SET_CONTEXT_PARAM.  In order to work around this wart, this
commit adds a proto-context struct which contains all the context create
parameters.

v2 (Daniel Vetter):
 - Better commit message
 - Use __set/clear_bit instead of set/clear_bit because there's no race
   and we don't need the atomics

v3 (Daniel Vetter):
 - Use manual bitops and BIT() instead of __set_bit

v4 (Daniel Vetter):
 - Add a changelog to the commit message
 - Better hyperlinking in docs
 - Create the default PPGTT in i915_gem_create_context

v5 (Daniel Vetter):
 - Hand-roll the initialization of UCONTEXT_PERSISTENCE

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708154835.528166-17-jason@jlekstrand.net