tests/xe_compute: Don't require ccs_mode on every GT

On platforms with standalone media, we expect some GTs to have CCS
engines (meaning ccs_mode can be tested if there's more than one CCS),
but the media GT will never have any CCS engines and will never expose
ccs_mode.  Move the igt_require() out of the loop so that we don't
declare the whole test a skip upon encountering the media GT when the
test already executed successfully on the primary GT.

v2:
 - Make the same fix for the compute_kernel subtest, not just the basic
   subtest.  (Gustavo)
 - Treat get_num_cslices()'s return value as a bool rather than an
   integer for clarity.

Cc: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Cc: Gustavo Sousa <gustavo.sousa@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>