drm/xe/rtp: Fix build error with clang < 21 and non-const initializers

drm/xe/rtp: Fix build error with clang < 21 and non-const initializers

Clang < 21 treats const-qualified compound literals at function scope as
having static storage duration, which requires all initializer elements
to be compile-time constants.  When xe_hw_engine.c initializes a local
struct xe_rtp_table_sr using XE_RTP_TABLE_SR(), the compound literals in
XE_RTP_TABLE_SR end up containing runtime values (e.g. blit_cctl_val
derived from gt->mocs.uc_index), triggering:

  xe_hw_engine.c:361: error: initializer element is not a compile-time constant
  xe_hw_engine.c:416: error: initializer element is not a compile-time constant

ARRAY_SIZE() cannot be used as a replacement because it expands through
__must_be_array() -> __BUILD_BUG_ON_ZERO_MSG() -> _Static_assert inside
sizeof(struct{}), which clang < 21 also rejects in the same context.

Replace ARRAY_SIZE() with an open-coded sizeof(arr)/sizeof(elem) in
XE_RTP_TABLE_SR and XE_RTP_TABLE to avoid both issues.

Fixes: e23fafb8594e ("drm/xe/rtp: Add struct types for RTP tables")
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Gustavo Sousa <gustavo.sousa@intel.com>
Cc: Violet Monti <violet.monti@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ashutosh Dixit <ashutosh.dixit@intel.com>
Cc: intel-xe@lists.freedesktop.org
Reported-by: Mark Brown <broonie@kernel.org>
Closes: https://lore.kernel.org/intel-xe/bfb0dee8-b243-47ba-a89d-71472b0d51c5@sirena.org.uk/
Assisted-by: GitHub_Copilot:claude-sonnet-4.6
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patch.msgid.link/20260605093305.110598-1-thomas.hellstrom@linux.intel.com
(cherry picked from commit a57011eff45e7265dc42a7adad68b84605d8f828)
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>