drm/i915: Move i915_gpu_error into its own header

Error state management code was moved into separate .c unit
but we didn't move related definitions into own header.

v2: move also intel_display_error_state forward decl
    fix ("Prefer 'unsigned int' to bare use of 'unsigned'")
    warnings detected by checkpatch in moved code (Michal)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180308095037.18264-5-michal.wajdeczko@intel.com