drm: Sanitize DRM_IOCTL_MODE_CREATE_DUMB input

Some drivers erroneously treat the .pitch and .size fields of struct
drm_mode_create_dumb as inputs. While the include/uapi/drm/drm_mode.h
header has a comment denoting them as outputs, that seemingly wasn't
enough to make drivers use them properly.

The result is that some userspace doesn't explicitly zero out those
fields, assuming that the kernel won't use them. That causes problems
since the data within the structure might be uninitialized, so bogus
data may end up confusing drivers (ridiculously large values for the
pitch, ...).

This series attempts to improve the situation by fixing all drivers to
not use the output fields. Furthermore to spare new drivers this bad
surprise, the DRM core now zeros out these fields prior to handing the
data structure to the driver.

Lessons learned from this are that future IOCTLs should be properly
documented (in the DRM DocBook for example) and should be rigorously
defined. To prevent misuse like this, userspace should be required to
zero out all output fields. The kernel should check for this and fail
if that's not the case.
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