display: add getter/setter for white balance

display: add getter/setter for white balance

Add a getter and setter function of display white balance which controls RGB gain and offset.

White balancing is adjusting standard point of white color, this because the light source is different for each device.
gain value is a value to be multiplied for RGB calculation.
offset value is a value to be added for RGB calculation.

These are function prototype to be added.
int device_display_set_white_balance(int display_index, display_white_balance_e white_balance_type, int value);
int device_display_get_white_balance(int display_index, display_white_balance_e white_balance_type, int *value);

With this enum type, we can get and set RGB gain and offset value for white balancing.
It is used as a parameter of getter and setter.
typedef enum
{
	DISPLAY_WHITE_BALANCE_R_GAIN = 0, /** White balance R Gain */
	DISPLAY_WHITE_BALANCE_G_GAIN, /** White balance G Gain */
	DISPLAY_WHITE_BALANCE_B_GAIN, /** White balance B Gain */
	DISPLAY_WHITE_BALANCE_R_OFFSET, /** White balance R Offset */
	DISPLAY_WHITE_BALANCE_G_OFFSET, /** White balance G Offset */
	DISPLAY_WHITE_BALANCE_B_OFFSET, /** White balance B Offset */
} display_white_balance_e;

Change-Id: Ie0adc74e76bcc8e8324767448aa03e3cce585af4
Signed-off-by: Yunhee Seo <yuni.seo@samsung.com>