drm/i915: Add support for DRRS in intel_dp_set_m_n

drm/i915: Add support for DRRS in intel_dp_set_m_n

Till Gen 7 we have two sets of M_N registers, but Gen 8 onwards
we have only one M_N register set. To support DRRS on both scenarios
a input parameter to intel_dp_set_m_n is added.

In case of DRRS, When platform provides two set of M_N registers for dp,
we can program them with two different dividers and switch between them.
But when only one such register set is provided, we have to program
the required divider M_N value on that registers itself.

Two enum members M1_N1 and M2_N2 are defined to represent the above
scenarios.

M1_N1        :	Program dp_m_n on M1_N1 registers
			dp_m2_n2 on M2_N2 registers (If supported)

M2_N2        :	Program dp_m2_n2 on M1_N1 registers
			M2_N2 registers are not supported

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>