drm/i915/gt: remove some limited use register access wrappers

Remove rmw_set(), rmw_clear(), clear_register(), rmw_set_fw(), and
rmw_clear_fw(). They're just one too many levels of abstraction for
register access, for very specific purposes.

clear_register() seems like a micro-optimization bypassing the write
when the register is already clear, but that trick has ceased to work
since commit 06b975d5 ("drm/i915: make intel_uncore_rmw() write
unconditionally"). Just clear the register in the most obvious way.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221123164916.4128733-1-jani.nikula@intel.com