drm/xe/guc: Use proper flag definitions when registering context

In H2G action context type is specified in flags dword in bits 2:1.
Use generic FIELD_PREP macro instead of misleading BIT logic.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250908123025.747-3-michal.wajdeczko@intel.com