drm/i915/display: Make POWER_DOMAIN_*() always result in enum intel_display_power_domain

drm/i915/display: Make POWER_DOMAIN_*() always result in enum intel_display_power_domain

In the hope of contributing to type safety in our code, let's ensure
that the type returned by the POWER_DOMAIN_*() macros is always of type
enum intel_display_power_domain.

v2:
  - Remove accidental +1 in definition of POWER_DOMAIN_PIPE(). (Jani)

Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250227-improve-type-safey-power-domain-macros-v3-2-b6eaa00f9c33@intel.com
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>