drm/xe/oa: Fix offset alignment for MERT WHITELIST_OA_MERT_MMIO_TRG

drm/xe/oa: Fix offset alignment for MERT WHITELIST_OA_MERT_MMIO_TRG

'head' argument for WHITELIST_OA_MERT_MMIO_TRG was previously wrong (not
multiple of 16). Fix this.

Fixes: ec02e49f21bc ("drm/xe/rtp: Whitelist OAMERT MMIO trigger registers")
Cc: stable@vger.kernel.org
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patch.msgid.link/20260629172634.1100983-1-ashutosh.dixit@intel.com
(cherry picked from commit f6c23e4589bdc69a5d2f79aed5c5bddd5d406cbe)
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>