drm/i915: Don't give up waiting on INVALID_MODE

Our current logic to read LSPCON's current mode, stops retries and
breaks wait-loop, if it gets LSPCON_MODE_INVALID as return from the
core function. This doesn't allow us to try reading the mode again.

This patch removes this condition and allows retries reading
the currnt mode until timeout.

This also fixes/prevents some of the noise in form of debug messages
while running IGT CI test cases.

V2: rebase, added r-b
V2: changed some debug message levels from debug->error and
    error->debug in lspcon_get_current_mode function.
V3: Rebase

Cc: Imre Deak <imre.deak@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102294
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102295
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102359
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103186


Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Mahesh Kumar <Mahesh1.kumar@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1507630064-17908-3-git-send-email-shashank.sharma@intel.com