drm/i915: Don't calculate a new clock in ILK+ code if it is already set

Remove the clock calculation from ironlake_crtc_compute_clock() when the
encoder compute_config() already set one. The value was just thrown away
in that case.

Note that the previously set clock is not validated against the limits
anymore. That is ok since the fixed clocks from DP and SDVO are within
the supported range, so the call to ironlake_compute_clocks() would
never fail in that case.

v2: Add note about not checking fixed clocks agains limits. (Maarten)
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458576016-30348-7-git-send-email-ander.conselvan.de.oliveira@intel.com