drm/xe: Convert mem_access assertion towards the runtime_pm state

The mem_access helpers are going away and getting replaced by
direct calls of the xe_pm_runtime_{get,put} functions. However, an
assertion with a warning splat is desired when we hit the worst
case of a memory access with the device really in the 'suspended'
state.

Also, this needs to be the first step. Otherwise, the upcoming
conversion would be really noise with warn splats of missing mem_access
gets.

v2: Minor doc changes as suggested by Matt

Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240222163937.138342-2-rodrigo.vivi@intel.com