drm/i915: Move the HSW power well flags to a common bitfield

Save some space by grouping the HSW power well descriptor flags along
with other flags in one bitfield.

This change also lets simplifying the definition of power well
descriptors sharing the same flags in an upcoming patch.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220414210657.1785773-5-imre.deak@intel.com