drm/xe: Prefer single underscore for header guards

Keep header guards consistent with regard to ifdef used. Prefer the more
commonly used in the driver.

$ git grep "ifndef __XE_" -- drivers/gpu/drm/xe | wc -l
8
$ git grep "ifndef _XE_" -- drivers/gpu/drm/xe | wc -l
112

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drm/xe: Fix kunit integration due to missing prototypes

In order to avoid -Werror=missing-prototypes, add the prototypes
in a separate tests/<test-name>_test.h file that is included by both
the implementation (tests/xe_<testname>.c, injected in xe.ko) and the
kunit module (tests/xe_<testname>_test.c -> xe-<testname>-test.ko).

v2: Add header and don't add ifdef to files that are already not built
when not using kunit (Matt Auld)

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>