drm/i915: move intel_hws_csb_write_index() out of i915_drv.h

Underscore prefix the index macros, and place
INTEL_HWS_CSB_WRITE_INDEX() as a macro next to them, to declutter
i915_drv.h.

v2: Don't underscore the index macros (Tvrtko)

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220209131143.3365230-1-jani.nikula@intel.com