drm: rework SET_MASTER and DROP_MASTER perm handling

This commit reworks the permission handling of the two ioctls. In
particular it enforced the CAP_SYS_ADMIN check only, if:
 - we're issuing the ioctl from process other than the one which opened
the node, and
 - we are, or were master in the past

This ensures that we:
 - do not regress the systemd-logind style of DRM_MASTER arbitrator
 - allow applications which do not use systemd-logind to drop their
master capabilities (and regain them at later point) ... w/o running as
root.

See the comment above drm_master_check_perm() for more details.

v1:
 - Tweak wording, fixup all checks, add igt test

v2:
 - Add a few more comments, grammar nitpicks.

Cc: Adam Jackson <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Pekka Paalanen <[email protected]>
Testcase: igt/core_setmaster/master-drop-set-user
Signed-off-by: Emil Velikov <[email protected]>
Reviewed-by: Adam Jackson <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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