drm/i915/cnl: Fix the CURSOR_COEFF_MASK used in DDI Vswing Programming

The Cursor Coeff is lower 6 bits in the PORT_TX_DW4 register
and hence the CURSOR_COEFF_MASK should be (0x3F << 0)

Fixes: 04416108

 ("drm/i915/cnl: Add registers related to voltage
swing sequences.")
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1498785241-21138-1-git-send-email-manasi.d.navare@intel.com

(cherry picked from commit fcace3b9

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>