drm/i915/display: Allow PSR2 selective fetch to be enabled at run-time

Right now CI is blacklisting module reload, so we need to be able to
enable PSR2 selective fetch in run time to test this feature before
enable it by default.
Changes in IGT will also be needed.

v2:
- Fixed handling of I915_PSR_DEBUG_ENABLE_SEL_FETCH in
intel_psr_debug_set()

Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209205036.351076-1-jose.souza@intel.com