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 value calculation.
offset vealue is a value to be added for RGB value calculation.

These are function prototype to be added.
int hal_device_display_set_white_balance(enum hal_display_white_balance, int value);
int hal_device_display_get_white_balance(enum hal_display_white_balance, int* value);

With this enum type, it is possible to get and set RGB gain and offset value for white balancing.
It is used as a parameter of HAL getter and setter function.
enum hal_display_white_balance {
	HAL_DISPLAY_WHITE_BALANCE_R_GAIN,
	HAL_DISPLAY_WHITE_BALANCE_G_GAIN,
	HAL_DISPLAY_WHITE_BALANCE_B_GAIN,
	HAL_DISPLAY_WHITE_BALANCE_R_OFFSET,
	HAL_DISPLAY_WHITE_BALANCE_G_OFFSET,
	HAL_DISPLAY_WHITE_BALANCE_B_OFFSET,
};

Change-Id: I3afe3953e15b61657a8dfd842a30fde553359586
Signed-off-by: Yunhee Seo <yuni.seo@samsung.com>
(cherry picked from commit e98384a40fc521aa8c07c751cf58e29eb0f58984)