drm/i915/gvt: treat intel_gvt_mpt as const in gvt code

The current interface of intel_gvt_register_hypervisor() expects a
non-const pointer to struct intel_gvt_mpt, even though the mediator
never modifies (or should modifiy) the content of this struct.

Change the function signature and relevant struct members to const to
properly express the API's intent and allow instances of intel_gvt_mpt
to be allocated as const.

While I was here, I also made KVM's instance of this struct const to
reduce the number of writable function pointers in the kernel.

Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: intel-gvt-dev@lists.freedesktop.org
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Julian Stecklina <julian.stecklina@cyberus-technology.de>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20201111172811.558443-1-julian.stecklina@cyberus-technology.de