drm/i915: Add i915_lpsp_capability debugfs

New i915_pm_lpsp igt solution approach relies on connector specific
debugfs attribute i915_lpsp_capability, it exposes whether an output is
capable of driving lpsp.

v2:
- CI fixup.
v3:
- register i915_lpsp_info only for supported connector. [Jani]
- use intel_display_power_well_is_enabled() instead of looking
  inside power_well count. [Jani]
- fixes the lpsp capable conditional logic. [Jani]
- combined the lpsp capable and enable info. [Jani]
v4:
- Separate out connector based debugfs i915_lpsp_capability
  lpsp enable status would be exposes by different entry. [Animesh]
v5:
- Add Platform Gen condition to add i915_lpsp_capability
  and some cosmetic nitpick changes. [Animesh]

Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415170555.15531-3-anshuman.gupta@intel.com