drm/xe: Avoid unconditional VRAM reads in H2G path

desc_read() issues an VRAM read which serializes the CPU and drains
posted writes on dGPU platforms. The H2G tracepoint evaluated its
arguments unconditionally, so even with tracing disabled the submission
path paid the full VRAM readf latency. Guard the tracepoint with
trace_xe_guc_ctb_h2g_enabled().

Adso move the descriptor status verification under CONFIG_DRM_XE_DEBUG.
This removes another unnecessary VRAM read in non-debug builfds.

This results in ~10× faster H2G submission and significantly reduces
lock contention across the driver.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Stuart Summers <stuart.summers@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patch.msgid.link/20260218043319.809548-3-matthew.brost@intel.com

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