drm/i915: Splitting intel_dp_detect

intel_dp_detect() is called for not just detection but
during modes enumeration as well. Repeating the whole
sequence during each of these calls is wasteful and
time consuming.
This patch moves probing for panel, DPCD read etc done in
intel_dp_detect() to a new function intel_dp_long_pulse().
Note that the behavior of intel_dp_detect() is changed to
report connected or disconnected depending on whether the
EDID is available or not.
This change will be required by further patches in the series
to avoid performing duplicated DPCD operations on hotplug.

v2: Moved a hunk to next patch of the series.
    Moved intel_dp_unset_edid to out. (Ander)

v3: Rephrased commit message and intel_dp_unset_dp() is called
    within intel_dp_set_dp() to free the previous EDID. (Ander)

v4: Added overriding of status to disconnected for MST. (Ander)

Tested-by: Nathan D Ciobanu <nathan.d.ciobanu@intel.com>
Signed-off-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Shubhangi Shrivastava <shubhangi.shrivastava@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
[anderco: fix parenthesis alignment]
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459341326-13142-1-git-send-email-shubhangi.shrivastava@intel.com