drm/i915: Runtime disable for eDP DRRS

Debugfs called i915_drrs_ctl is added to enable and disable the
eDP DRRS. Writing 0 will disable the feature, whereas non-zero
will enable the feature.

Possibility of disabling the DRRS, enables the testing of the
frontbuffer tracking based features (FBC, DRRS and PSR) as
standalone or any combination of the set.

[v2]: ctl interface is moved from module parameter to debugfs [Rodrigo]

Signed-off-by: C, Ramalingam <ramalingam.c@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1510079903-29441-1-git-send-email-ramalingam.c@intel.com