drm/vmwgfx: Validate command header size against SVGA_CMD_MAX_DATASIZE

drm/vmwgfx: Validate command header size against SVGA_CMD_MAX_DATASIZE

This data originates from userspace and is used in buffer offset
calculations which could potentially overflow causing an out-of-bounds
access.

Fixes: 8ce75f8ab904 ("drm/vmwgfx: Update device includes for DX device functionality")
Reported-by: Rohit Keshri <rkeshri@redhat.com>
Signed-off-by: Ian Forbes <ian.forbes@broadcom.com>
Reviewed-by: Maaz Mombasawala <maaz.mombasawala@broadcom.com>
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patch.msgid.link/20251021190128.13014-1-ian.forbes@broadcom.com