drm/i915/dsi: Using the bpp value wrt the pixel format

The bpp value which is used while calulating the txbyteclkhs values
should be wrt the pixel format value. Currently bpp is coming
from pipe config to calculate txbyteclkhs. Fix it in this patch.

V2: dsi_pixel_format_bpp is used to retrieve the bpp from pixel_format
	[Review: Jani]

Signed-off-by: Deepak M <m.deepak@intel.com>
Signed-off-by: Yogesh Mohan Marimuthu <yogesh.mohan.marimuthu@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Tested-by: Mika Kahola <mika.kahola@intel.com> # BYT
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455203007-10850-1-git-send-email-ramalingam.c@intel.com