drm/i915: pass status instead of enable flags to i915_enable_pipestat

There isn't any PSR interrupt enable bit for pipe A, so we couldn't
enable it through the current API. Passing the corresponding status bits
solves this and also makes the mapping between enable and status bits
simpler on VLV (addressed in an upcoming patch).

Except of checking for invalid status bit arguments, no functional
change.

v2: split out the low level parts of i915_enable_pipestat accepting
    separate enabled and status masks, to make the non-standard mapping
    between those masks stand out more (added in the next patch)
    (Jesse,Daniel)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>