drm/i915: Initialise entry in intel_ppat_get() for older compilers

gcc-4.7.3 is confused by the guards inside intel_ppat_get() and reports:

drivers/gpu/drm/i915/i915_gem_gtt.c: In function ‘intel_ppat_get’:
drivers/gpu/drm/i915/i915_gem_gtt.c:3044:27: warning: ‘entry’ may be used uninitialized in this function [-Wmaybe-uninitialized]

Forgive the compiler this once, and rearrange the code so that entry is
always initialised.

v2: Flavour with a bit of NULL (instead of ERR_PTR(-ENOSPC))

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171115131705.16341-1-chris@chris-wilson.co.uk


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>