drm/vmwgfx: Limit display layout ioctl array size to VMWGFX_NUM_DISPLAY_UNITS

Currently the array size is only limited by the largest kmalloc size which
is incorrect. This change will also return a more specific error message
than ENOMEM to userspace.

Signed-off-by: Ian Forbes <ian.forbes@broadcom.com>
Reviewed-by: Zack Rusin <zack.rusin@broadcom.com>
Reviewed-by: Martin Krastev <martin.krastev@broadcom.com>
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240808200634.1074083-1-ian.forbes@broadcom.com