drm/i915/edp: Allow alternate fixed mode for eDP if available.

Some fixed resolution panels actually support more than one mode,
with the only thing different being the refresh rate.  Having this
alternate mode available to us is desirable, because it allows us to
test PSR on panels whose setup time at the preferred mode is too long.
With this patch we allow the use of the alternate mode if it's
available and it was specifically requested.

v2 and v3: Rebase
v4: * Fix up some leaky mode stuff (Chris)
    * Rebase
v5: * Fix a NULL pointer derefrence (David Weinehall)
v6: * Whitespace / spelling / checkpatch clean-up; no functional
      change. (David)
    * Rebase

Cc: David Weinehall <david.weinehall@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: David Weinehall <david.weinehall@linux.intel.com>
Signed-off-by: Jim Bride <jim.bride@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1502308133-26892-1-git-send-email-jim.bride@linux.intel.com