drm/i915: Reorganizing intel_dp_check_link_status

drm/i915: Reorganizing intel_dp_check_link_status

When created originally intel_dp_check_link_status()
was supposed to handle only link training for short
pulse but has grown into handler for short pulse itself.
This patch cleans up this function by splitting it into
two halves. First intel_dp_short_pulse() is called,
which will be entry point and handle all logic for
short pulse handling while intel_dp_check_link_status()
will retain its original purpose of only doing link
status related work.

intel_dp_short_pulse: All existing code other than
link status read and link training upon error status.

intel_dp_check_link_status:
The link status should be read on short pulse
irrespective of panel being enabled or not so
intel_dp_get_link_status() performs dpcd read first
then based on crtc active / enabled it will
perform the link training.

This is because short pulse is a generic interrupt
which should always be handled, because it may mean:
	1. Hotplug/unplug of MST panel
	2. Hotplug/unplug of dongle
	3. Link status change for other DP panels

v2: Added WARN_ON to intel_dp_check_link_status()
    Removed a call to intel_dp_get_link_status() (Ander)

v3: Changed commit message to explain need of link status
    being read before performing encoder checks (Daniel)

v4: Changed commit message to explain need of reading
    link status on short pulse (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/1459341326-13142-3-git-send-email-shubhangi.shrivastava@intel.com