drm/i915: Implement vm_ops->access for gdb access into mmaps

gdb uses ptrace() to peek and poke bytes of the target's address space.
The driver must implement an vm_ops->access() handler or else gdb will
be unable to inspect the pointer and report it as out-of-bounds.
Worse than useless as it causes immediate suspicion of the valid GTT
pointer, distracting the poor programmer trying to find his bug.

v2: Write-protect readonly objects (Matthew).

Testcase: igt/gem_mmap_gtt/ptrace
Testcase: igt/gem_mmap_offset/ptrace
Suggested-by: Kristian H. Kristensen <[email protected]>
Signed-off-by: Chris Wilson <[email protected]>
Cc: Matthew Auld <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Maciej Patelczyk <[email protected]>
Cc: Kristian H. Kristensen <[email protected]>
Reviewed-by: Matthew Auld <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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