drm/i915: Add definitions for MI_MATH command

We can use it in i915 for updating parts of unmasked registers from
within a batch. We're also adding Gen8+ versions of CS_GPR registers
(aka MI_MATH_REG in the coprocessor).

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190926100635.9416-4-michal.winiarski@intel.com