drm/i915: Added debugfs support for PSR Status

Adding support for PSR Status, PSR entry counter and performance counters.
Heavily based on initial work from Shobhit.

v2: Fix PSR Status Link bits by Paulo Zanoni.
v3: Prefer seq_puts to seq_printf by Paulo Zanoni.
v4: Fix identation by Paulo Zanoni.
v5: Return earlier if it isn't Haswell in order to avoid reading non-existing
    registers - by Paulo Zanoni.

CC: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Credits-by: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>