Kill MMIO IOCTL

https://lore.kernel.org/all/Y9hw0sSC58B32yPg@mdroper-desk1.amr.corp.intel.com/

diff --git a/drivers/gpu/drm/xe/xe_mmio.c b/drivers/gpu/drm/xe/xe_mmio.c
...
+static const i915_reg_t mmio_read_whitelist[] = {

• RING_TIMESTAMP(RENDER_RING_BASE),
  +};

+int xe_mmio_ioctl(struct drm_device *dev, void *data,

•         struct drm_file *file)                                                                                                                                                                                                       

My understanding is that having a register read ioctl was pretty much
ancient history that was grandfathered into the i915 design. Is there
any reason to keep this on Xe rather than just providing a more
appropriate way to obtain the render timestamp (e.g., via the query
ioctl or something)? Among other things, userspace needs to interpret
the register value, which seems like something the kernel should really
be doing for it; it's possible new fields could be added to the register
in the future containing something we don't want to just blindly expose
to userspace.

very good point.
Also these registers can be read with MI_LRI commands from userspace directly.