Documentation/gpu: Clarify drm memory stats definition

Documentation/gpu: Clarify drm memory stats definition

Define how to handle buffers with multiple possible placement so we
don't get incompatible implementations. Callout the resident requirement
for drm-purgeable- explicitly. Remove the requirement for there to be
only drm-memory- or only drm-resident-, it's not what's implemented and
having both is better for back-compat. Also re-order the paragraphs to
flow better.

Signed-off-by: Yunxiang Li <Yunxiang.Li@amd.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241219151411.1150-4-Yunxiang.Li@amd.com
Signed-off-by: Christian König <christian.koenig@amd.com>