drm/i915: Rename struct i915_power_well field data to id

Calling it data seems to imply arbitrary data can be associated with the
power well. However, that field is used for look ups and expected to be
unique, so rename it.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/f3916c3c5bfa793b0fc870fd44007a3ff425194d.1475770848.git-series.ander.conselvan.de.oliveira@intel.com