drm/xe/pcode: Initialize data0 for pcode read routine

drm/xe/pcode: Initialize data0 for pcode read routine

There are two registers filled in when reading data from
pcode besides the mailbox itself. Currently, we allow a NULL
value for the second of these two (data1) and assume the first
is defined. However, many of the routines that are calling
this function assume that pcode will ignore the value being
passed in and so leave that first value (data0) defined but
uninitialized. To be safe, make sure this value is always
initialized to something (0 generally) in the event pcode
behavior changes and starts using this value.

v2: Fix sob/author

Signed-off-by: Stuart Summers <stuart.summers@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://lore.kernel.org/r/20250819201054.393220-1-stuart.summers@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>