drm/xe/rtp: Split rtp process initialization

The selection between hwe and gt is exposed to the outside of rtp, by
the xe_rtp_process() function. However it doesn't make seense from the
caller point of view to pass a hwe and a gt as argument since the gt
should always be the one containing the hwe.

This clarifies the interface by separating the context creation into an
initializer. The initializer then passes the correct value and there
should never be a case with hwe and gt set: when hwe is passed, the gt
is the one containing it. Internally the functions continue receiving
the argument separately.

v2: Leave the device-only context to a separate patch if they are indeed
    needed later

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20230526164358.86393-3-lucas.demarchi@intel.com


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