drm/i915/selftests: Tidy prng constructor for client blits

Since we only initialise the prng once within the scope of the selftest,
we can use the default initialiser.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201202130406.18461-1-chris@chris-wilson.co.uk