drm: Add DRM client cap for aspect-ratio

To enable aspect-ratio support in DRM, blindly exposing the aspect
ratio information along with mode, can break things in existing
non-atomic user-spaces which have no intention or support to use this
aspect ratio information.

To avoid this, a new drm client cap is required to enable a non-atomic
user-space to advertise if it supports modes with aspect-ratio. Based
on this cap value, the kernel will take a call on exposing the aspect
ratio info in modes or not.

This patch adds the client cap for aspect-ratio.

Since no atomic-userspaces blow up on receiving aspect-ratio
information, the client cap for aspect-ratio is always enabled
for atomic clients.

Cc: Ville Syrjala <[email protected]>
Cc: Shashank Sharma <[email protected]>
Signed-off-by: Ankit Nautiyal <[email protected]>

V3: rebase
V4: As suggested by Marteen Lankhorst modified the commit message
    explaining the need to use the DRM cap for aspect-ratio. Also,
    tweaked the comment lines in the code for better understanding and
    clarity, as recommended by Shashank Sharma.
V5: rebase
V6: rebase
V7: rebase
V8: rebase
V9: rebase
V10: rebase
V11: rebase
V12: As suggested by Daniel Vetter and Ville Syrjala,
     always enable aspect-ratio client cap for atomic userspaces,
     if no atomic userspace breaks on aspect-ratio bits.
V13: rebase
V14: rebase

Reviewed-by: Shashank Sharma <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]>
Signed-off-by: Maarten Lankhorst <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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