drm/xe: Use xe_mmio_read32() to read mtcfg register

drm/xe: Use xe_mmio_read32() to read mtcfg register

The mtcfg register is a 32-bit register and should therefore be
accessed using xe_mmio_read32().

Other 3 changes per codestyle suggestion:
"
xe_mmio.c:83: CHECK: Alignment should match open parenthesis
xe_mmio.c:131: CHECK: Comparison to NULL could be written "!xe->mmio.regs"
xe_mmio.c:315: CHECK: line length of 103 exceeds 100 columns
"

Fixes: dd08ebf6c352 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Reviewed-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Shuicheng Lin <shuicheng.lin@intel.com>
Link: https://lore.kernel.org/r/20250513153010.3464767-1-shuicheng.lin@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>