dri/kms: Always zero out struct drm_mode_create_dumb

dri/kms: Always zero out struct drm_mode_create_dumb

The DRM_IOCTL_MODE_CREATE_DUMB (and others) IOCTL isn't very rigorously
specified, which has the effect that some kernel drivers do not consider
the .pitch and .size fields of struct drm_mode_create_dumb outputs only.
Instead they will use these as lower bounds and overwrite them only if
the values that they compute are larger than what userspace provided.

This works if and only if userspace initializes the fields explicitly to
either 0 or some meaningful value. However, if userspace just leaves the
values uninitialized and the struct drm_mode_create_dumb is allocated on
the stack for example, the driver may try to overallocate buffers.

Fortunately most userspace does zero out the structure before passing it
to the IOCTL, but there are rare exceptions. Mesa is one of them. In an
attempt to rectify this situation, kernel drivers are being updated to
not use the .pitch and .size fields as inputs. However in order to fix
the issue with older kernels, make sure that Mesa always zeros out the
structure as well.

Future IOCTLs should be more rigorously defined so that structures can
be validated and IOCTLs rejected if output fields aren't set to zero.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>