drm/i915/psr: Refactor psr short pulse handler

eDP spec states that when sink enconters a problem that prevents it
to keep PSR running it should set PSR status to internal error and
set the reason why it happen to PSR_ERROR_STATUS but it is not how it
was implemented.
But also I don't want to change this behavior, who knows if there is
a panel out there that only set the PSR_ERROR_STATUS.

So here refactoring the code a bit to make more easy to read what was
state above as more checks will be added to this function.

v2:
returning a int instead of a bool in psr_get_status_and_error_status()

Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191128014852.214135-2-jose.souza@intel.com