drm/i915: Sharing the pixel_format_from_vbt to whole i915

Shared the function pixel_format_from_vbt for whole display module.
Function declaration is added to intel_dsi.h.

V2: Moved the function to intel_dsi.c and renamed as per the purpose
	of the function. Suggested by Jani.

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>

Previously reviewed at https://lists.freedesktop.org/archives/intel-gfx/2016-April/091736.html
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460019967-26501-1-git-send-email-ramalingam.c@intel.com