drm/gma500: Sanity-check pipe index

drm/gma500: Sanity-check pipe index

If the DSI output isn't connected, then mdfld_dsi_encoder_get_pipe()
will return -1. The mdfld_dsi_dp_mode_set() function doesn't properly
check for this condition and causes the following compiler warnings:

	  CC      drivers/gpu/drm/gma500/mdfld_dsi_dpi.o
	drivers/gpu/drm/gma500/mdfld_dsi_dpi.c: In function ‘mdfld_dsi_dpi_mode_set’:
	drivers/gpu/drm/gma500/mdfld_dsi_dpi.c:828:35: warning: array subscript is below array bounds [-Warray-bounds]
	  u32 pipeconf = dev_priv->pipeconf[pipe];
	                                   ^
	drivers/gpu/drm/gma500/mdfld_dsi_dpi.c:829:33: warning: array subscript is below array bounds [-Warray-bounds]
	  u32 dspcntr = dev_priv->dspcntr[pipe];
	                                 ^

Fix this by checking for a valid pipe before indexing the pipeconf and
dspcntr arrays.

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450178476-26284-2-git-send-email-boris.brezillon@free-electrons.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>