drm/i915/error: standardize function style in error capture

some of the static functions used from capture() have the "i915_"
prefix while other don't; most of them take i915 as a parameter, but one
of them derives it internally from error->i915. Let's be consistent by
avoiding prefix for static functions and by getting i915 from
error->i915. While at it, s/dev_priv/i915 in functions that don't
perform register reads.

v2: take i915 from error->i915 (Michal), s/dev_priv/i915,
    update commit message

Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180305222122.3547-2-daniele.ceraolospurio@intel.com


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>