gpu: drm: i915: remove dead code

Local variable has_reduced_clock is assigned to a constant value and it is
never updated again. Remove this variable and the dead code it guards.

Addresses-Coverity-ID: 1362230
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170515215605.GA14963@embeddedgus