drm/i915: State readout and cross-checking for dp_m2_n2

drm/i915: State readout and cross-checking for dp_m2_n2

Adding relevant read out comparison code, in check_crtc_state, for the new
member of crtc_config, dp_m2_n2, which was introduced to store link_m_n
values for a DP downclock mode (if available). Suggested by Daniel.

v2: Changed patch title.
Daniel's review comments incorporated.
Added relevant state readout code for M2_N2. dp_m2_n2 comparison to be done
only when high RR is not in use (This is because alternate m_n register
programming will be done only when low RR is being used).

v3: Modified call to get_m2_n2 which had dp_m_n as param by mistake.
Compare dp_m_n and dp_m2_n2 for gen 7 and below. compare the structures
based on DRRS state for gen 8 and above.
Save and restore M2 N2 registers for gen 7 and below

v4: For Gen>=8, check M_N registers against dp_m_n and dp_m2_n2 as there is
only one set of M_N registers

v5: Removed the chunk which saves and restores M2_N2 registers. Modified
get_m_n() to get M2_N2 registers as well. Modified the macro which compares
hw.dp_m_n against sw.dp_m2_n2/sw.dp_m_n for gen > 8.

v6: Added check to compare dp_m2_n2 only when DRRS is enabled

v7: Modified drrs check to use has_drrs

v8: Add has_drrs check before reading M2_N2 registers

Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>