drm/i915: Cleaning up intel_dp_hpd_pulse

Current DP detection has DPCD operations split across
intel_dp_hpd_pulse and intel_dp_detect which contains
duplicates as well. Also intel_dp_detect is called
during modes enumeration as well which will result
in multiple dpcd operations. So this patch tries
to solve both these by bringing all DPCD operations
in one single function and make intel_dp_detect
use existing values instead of repeating same steps.

v2: Pulled in a hunk from last patch of the series to
    this patch. (Ander)

v3: Added MST hotplug handling. (Ander)

v4: Added a flag to check if detect is performed to
    prevent multiple detects on hotplug. (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 aligment]
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459341326-13142-2-git-send-email-shubhangi.shrivastava@intel.com