drm/dsi: add two DSI mode flags for BLLP

Add two more dsi mode_flags. MIPI_DSI_MODE_VIDEO_BLLP
means DSI is allowed to enter low power stop mode
during BLLP period, and MIPI_DSI_MODE_VIDEO_EOF_BLLP
means DSI is allowed to enter low power stop  mode in
the BLLP area of the last line of a frame.

CRs-Fixed: 2140653
Change-Id: Idf1550a4d34be17e74301a65fdccf5a9e145598b
Signed-off-by: Ray Zhang <rayz@codeaurora.org>