drm/i915/gtt: Replace struct_mutex serialisation for allocation

Instead of relying on the caller holding struct_mutex across the
allocation, push the allocation under a tree of spinlocks stored inside
the page tables. Not only should this allow us to avoid struct_mutex
here, but it will allow multiple users to lock independent ranges for
concurrent allocations, and operate independently. This is vital for
pushing the GTT manipulation into a background thread where dependency
on struct_mutex is verboten, and for allowing other callers to avoid
struct_mutex altogether.

v2: Restore lost GEM_BUG_ON for removing too many PTE from
gen6_ppgtt_clear_range.

Signed-off-by: Chris Wilson <[email protected]>
Cc: Matthew Auld <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Reviewed-by: Joonas Lahtinen <[email protected]>
Acked-by: Mika Kuoppala <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]