drm/xe/tests: Fix g2g_test_array indexing

drm/xe/tests: Fix g2g_test_array indexing

The G2G KUnit test allocates a compact N×N
matrix sized by gt_count and verifies entries
using dense indices: idx = (j * gt_count) + i

The producer path currently computes idx using
gt->info.id. However, gt->info.id values
are not guaranteed to be contiguous.
For example, with gt_count=2 and IDs {0,3},
this formula produces indices beyond the
allocated range, causing mismatches and
potential out-of-bounds access.

Update the producer to map each GT to a dense
index in [0..gt_count-1] and compute:
    idx = (tx_dense * gt_count) + rx_dense

Additionally, introduce an event-based delay
in g2g_test_in_order() to ensure ordering
between sends.

v2: Add single helper function (Daniele)

v3: Modify comment (Daniele)

Signed-off-by: Pallavi Mishra <pallavi.mishra@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Link: https://patch.msgid.link/20260129054722.2150674-1-pallavi.mishra@intel.com