drm/i915: Add MIPI_IO WA and program DSI regulators

Enable MIPI IO WA for BXT DSI as per bspec and
program the DSI regulators.

v2: Moved IO enable to pre-enable as per Mika's
review comments. Also reused the existing register
definition for BXT_P_CR_GT_DISP_PWRON.

v3: Added Programming the DSI regulators as per disable/enable
sequences.

v4: Restricting regulator changes to BXT as suggested by
Jani/Mika

v5: Removed redundant read/modify for regulator register as
per Jani's comment. Maintain enable/disable symmetry as per spec.

Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Vidya Srinivas <vidya.srinivas@intel.com>
Acked-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1485353603-11260-1-git-send-email-vidya.srinivas@intel.com