drm/i915/hwmon: Get mutex and rpm ref just once in hwm_power_max_write

In preparation for follow-on patches, refactor hwm_power_max_write to take
hwmon_lock and runtime pm wakeref at start of the function and release them
at the end, therefore acquiring these just once each.

Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230420164041.1428455-2-ashutosh.dixit@intel.com