drm/i915/display: Introduce new intel_psr_pause/resume function

This introduces the following function that can exit and activate a psr
source when intel_psr is already enabled.

- intel_psr_pause(): Pause current PSR. It deactivates current psr state.
- intel_psr_resume(): Resume paused PSR. It activates paused psr state.

v2: Address Jose's review comment.
  - Remove unneeded changes around the intel_psr_enable().
  - Add intel_psr_post_exit() which processes waiting until PSR is idle
    and WA for SelectiveFetch.
v3: Address Jose's review comment.
  - Rename intel_psr_post_exit() to intel_psr_wait_exit_locked().
  - Move WA_1408330847 to intel_psr_disable_locked()
  - If the PSR is paused by an explicit intel_psr_paused() call, make the
    intel_psr_flush() not to activate PSR.
v4: Address Jose's review comment.
  - In order to avoid the scenario of PSR is not active but there is a
    scheduled psr->work, it changes the check routine of intel_psr_pause()
    for PSR's enablement from "psr->active" to "psr->enable".

Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210608085415.515342-1-gwan-gyeong.mun@intel.com