drm/i915: fix comment on I915_{READ, WRITE}_FW

Comment mentioned use of intel_uncore_forcewake_irq{unlock, lock}
functions which are nonexistent (and never were).

The description was also incomplete and could cause confusion. Updated
comment is more elaborate on usage and caveats.

v2: mention __locked variant of intel_uncore_forcewake_{get,put} instead
    of plain ones

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilsono.c.uk>
[Mika: removed two superfluous lines on comment noted by Chris]
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1477399682-3133-1-git-send-email-arkadiusz.hiler@intel.com