drm/i915: Refactor PSR status debugfs

The old debugfs fields was not following a naming partern and it was
a bit confusing.

So it went from:
~$ sudo more /sys/kernel/debug/dri/0/i915_edp_psr_status
Sink_Support: yes
PSR mode: PSR1
Enabled: yes
Busy frontbuffer bits: 0x000
Main link in standby mode: no
HW Enabled & Active bit: yes
Source PSR status: 0x24050006 [SRDONACK]

To:
~$ sudo more /sys/kernel/debug/dri/0/i915_edp_psr_status
Sink support: yes [0x03]
PSR mode: PSR1 enabled
Source PSR ctl: enabled [0x81f00e26]
Source PSR status: IDLE [0x04010006]
Busy frontbuffer bits: 0x00000000

The 'Main link in standby mode' was removed as it is not useful but
if needed by someone the information is still in the register value
of 'Source PSR ctl' inside of the brackets, PSR mode and Enabled was
squashed into PSR mode, some renames and reorders and we have this
cleaner version. This will also make easy to parse debugfs for IGT
tests.

v2: Printing sink PSR version with only 2 hex digits as it is a byte

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Suggested-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190117205548.28378-2-jose.souza@intel.com